Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Common shares Common Stock [Member] Preferred Shares Preferred Stock [Member] Equity Components Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Additional Paid-in Capital Additional Paid-in Capital [Member] Accumulated Dividends in Excess of Net Income Accumulated Distributions in Excess of Net Income [Member] Accumulated Other Comprehensive Income/(Loss) AOCI Attributable to Parent [Member] Noncontrolling Interests Noncontrolling Interest [Member] Statement [Line Items] Statement [Line Items] Contributions from noncontrolling interests Contributions from noncontrolling interests Contributions from noncontrolling interests Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning balance (in shares) Shares, Outstanding Beginning balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest January 1 adoption of new accounting standard - See Note 2 Cumulative Effect on Retained Earnings, Net of Tax Net income, excluding amount attributable to redeemable noncontrolling interests Profit Loss Excluding Redeemable Noncontrolling Interests Net income (loss) excluding portion attributable to redeemable noncontrolling interests. Other comprehensive (loss) income - change in value of interest rate swaps Other Comprehensive Income (Loss), Net of Tax Dividends declared to common shareholders Dividends, Common Stock, Cash Dividends declared to preferred shareholders Dividends, Preferred Stock, Cash Distributions declared to noncontrolling interests Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Shares issued (in shares) Stock Issued During Period, Shares, New Issues Shares issued Stock Issued During Period, Value, New Issues Exercise of stock options (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Exercise of stock options Stock Issued During Period, Value, Stock Options Exercised Shares issued under dividend reinvestment plan (in shares) Stock Issued During Period, Shares, Dividend Reinvestment Plan Shares issued under dividend reinvestment plan Stock Issued During Period, Value, Dividend Reinvestment Plan Share-based compensation expense, net of forfeitures (in shares) Share Based Compensation Expense Net Shares Number of shares issued during the period as a result of our long term incentive, bonus or other share based compensation plans, net of restricted share award forfeitures. Share-based compensation expense, net of forfeitures Share Based Compensation Expense Net Value This element represents the amount of recognized equity-based compensation related to other long-term incentive and bonus plans during the period, that is, the amount recognized as expense in the income statement (or as asset if compensation is capitalized). Shares withheld for employee taxes (in shares) Shares Paid for Tax Withholding for Share Based Compensation Shares withheld for employee taxes Adjustments Related to Tax Withholding for Share-based Compensation Conversion and redemption of OP units (in shares) Conversion and Redemption of Operating Partnership Units in Shares Number of shares issued during the period as a result of the conversion and redemption of operating partnership units into shares Conversion and redemption of OP units Conversion And Redemption Of Operating Partnership Units Decrease in noncontrolling interest as a result of redeeming, purchasing and/or converting to common stock the interests of Operating Partnership Units. Purchase of noncontrolling interests Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Adjustment to redeemable noncontrolling interests Stockholders' Equity, Other Ending balance (in shares) Ending balance Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Finite-Lived Intangible Assets [Table Text Block] Schedule of Finite-Lived Intangible Assets [Table Text Block] Finite-lived Intangible Assets Amortization Expense [Table Text Block] Finite-lived Intangible Assets Amortization Expense [Table Text Block] Amortization for acquired in-place leases during the next five years and thereafter Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Commitments and Contingencies Disclosure [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Litigation Status [Axis] Litigation Status [Axis] Litigation Status [Domain] Litigation Status [Domain] Pending Litigation Pending Litigation [Member] Name of Property Name of Property [Axis] Name of Property [Domain] Name of Property [Domain] Santana Row Santana Row [Member] Santana Row Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] gain on sale of real estate gain on sale of real estate [Member] gain on sale of real estate [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Member One In Plaza El Segundo Member One In Plaza El Segundo [Member] Member One In Plaza El Segundo [Member] Member Two In Plaza El Segundo Member Two In Plaza El Segundo [Member] Member Two In Plaza El Segundo [Member] Contractual Obligation [Axis] Contractual Obligation [Axis] Contractual Obligation [Domain] Contractual Obligation [Domain] Capital improvement, development, and redevelopment projects Capital Improvement, Development, And Redevelopment Projects [Member] Capital Improvement, Development, And Redevelopment Projects [Member] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Scenario, Forecast Scenario, Forecast [Member] Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Melville Mall MelvilleMall [Member] MelvilleMall [Member] Real Estate Property Ownership [Axis] Real Estate Properties [Domain] Mercer Mall Mercer Mall [Member] Mercer Mall [Member] Congressional Plaza Congressional Plaza [Member] Congressional Plaza [Member] Montrose Crossing Montrose Crossing [Member] Montrose Crossing [Member] Plaza El Segundo Plaza El Segundo [Member] Plaza El Segundo [Member] The Grove at Shrewsbury The Grove at Shrewsbury [Member] The Grove at Shrewsbury [Member] Brook 35 Brook 35 [Member] Brook 35 [Member] Grove at Shrewsbury and Brook 35 Grove At Shrewsbury And Brook 35 [Member] GroveatShrewsbury & Brook 35 [Member] Option By Type [Axis] Option By Type [Axis] Option By Type [Domain] Option By Type [Domain] Put Option Put Option [Member] Range [Axis] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Loss Contingencies [Line Items] Settlement amount Litigation Settlement, Amount Awarded to Other Party Potential remaining liability (less than) Loss Contingency, Estimate of Possible Loss Reversal of warranty reserve Standard and Extended Product Warranty Accrual, Period Increase (Decrease) Reserves for warranties and general liability Standard and Extended Product Warranty Accrual Payments from reserves Standard and Extended Product Warranty Accrual, Decrease for Payments Letters of credit outstanding, amount Letters of Credit Outstanding, Amount Contractual obligations Contractual Obligation Purchase price under option Purchase Price Under Option Purchase Price Under Option Maximum partnership percentage required to purchase Option Indexed to Issuer's Equity, Percentage Of Indexed Ownership Interest Option Indexed to Issuer's Equity, Percentage Of Indexed Ownership Interest Indexed ownership interest percentage Ownership Interest of Other Partner in Put Option The percentage of partnership interest we are required to purchase in a put option. Estimated maximum liability upon exercise of the put option Estimated Liability Upon Exercise Of Option Estimated Liability Upon Exercise Of Option Number of members with purchase options Number of Members With Purchase Options Number of Members With Purchase Options Payments acquire noncontrolling interests Payments to Noncontrolling Interests DownREIT operating partnership units, outstanding Partners' Capital Account, Units Operating partnership units outstanding, fair value Operating Partnership Units Fair Value Fair value of operating partnership units at the balance sheet date. Components Of Rental Income and Expense [Abstract] Components Of Rental Income And Expense [Abstract] COMPONENTS OF RENTAL INCOME AND EXPENSE Operating Leases Revenue And Expense Text Block The principal components of rental income and/or rental expense recognized for the period. Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Restricted cash Restricted Cash and Cash Equivalents Total cash, cash equivalents, and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Quarterly Financial Information Disclosure [Abstract] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Land Land [Member] Condominiums Condominiums [Member] Condominiums [Member] Residential Residential Real Estate [Member] Assembly Row and Pike & Rose Assembly Row and Pike & Rose [Member] Assembly Row and Pike & Rose [Member] Chelsea Chelsea [Member] Chelsea [Member] Atlantic Plaza Atlantic Plaza [Member] Atlantic Plaza Assembly Row Assembly Square Market Place Assembly Row [Member] Assembly Square Market Place Assembly Row [Member] 150 Post Street And North Lake Commons 150 Post Street And North Lake Commons [Member] 150 Post Street And North Lake Commons [Member] Bethesda Row Bethesda Row [Member] Bethesda Row [Member] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Gain on sale Gain (Loss) on Sale of Properties Gain on sale of real estate Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes Early extinguishment of debt Gain (Loss) on Extinguishment of Debt Early extinguishment of debt charge Payment for Debt Extinguishment or Debt Prepayment Cost Accounting Policies [Abstract] Minimum percentage of taxable income distributed to shareholders Minimum Percentage Of Taxable Income Distributed To Shareholders Minimum percentage of taxable income distributed to shareholders. Material unrecognized tax benefits Unrecognized Tax Benefits Number of properties exceeding segment reporting threshold Number of Properties Exceeding Segment Reporting Threshold Number of Properties Exceeding Segment Reporting Threshold Segment reporting threshold Segment Reporting, Factors Used to Identify Entity's Reportable Segments Number of segments Number of Reportable Segments SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation Disclosure [Abstract] Schedule III - Summary of Real Estate and Accumulated Depreciation SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation Disclosure [Text Block] Subsequent Events [Abstract] Statement of Cash Flows [Abstract] OPERATING ACTIVITIES Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Adjustment Depreciation And Amortization The cash flow addback for the non-cash expense of depreciation and amortization including depreciation and amortization related to discontinued operations. Gain on sale of real estate and change in control of interests, net Gain (loss) on change in control of interests and sale of real estate Gain (loss) on change in control of interests and sale of real estate Early extinguishment of debt Loss (income) from real estate partnerships Income (Loss) from Equity Method Investments Other, net Other Noncash Income (Expense) Changes in assets and liabilities, net of effects of acquisitions and dispositions: Increase (Decrease) in Operating Capital [Abstract] Proceeds from new market tax credit transaction, net of deferred costs Proceeds from new market tax credit transaction, net of deferred costs Proceeds from new market tax credit transaction, net of deferred costs Decrease in accounts receivable, net Increase (Decrease) in Accounts and Other Receivables (Increase) decrease in prepaid expenses and other assets Increase (Decrease) in Prepaid Expense and Other Assets Increase in accounts payable and accrued expenses Increase (Decrease) in Accounts Payable and Accrued Liabilities Increase (decrease) in security deposits and other liabilities Increase Decrease In Security Deposits And Other Liabilities The net change in Security Deposits And Other Liabilities which includes security deposits, deferred revenue and prepaid rent. Also includes liabilities not individually reported in the financial statements, or not separately disclosed in notes. Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities INVESTING ACTIVITIES Net Cash Provided by (Used in) Investing Activities [Abstract] Acquisition of real estate Payments to Acquire Real Estate Capital expenditures - development and redevelopment Capital Expenditures Development Redevelopment Payments to develop or redevelop real estate assets is the process of adding improvements on or to a parcel of land. Such improvements may include drainage, utilities, subdividing, access, buildings, and any combination of these elements. In addition, the cash outflow from construction costs to date on capital projects that have not been completed and assets being constructed that are not ready to be placed into service. Capital expenditures - other Payments for Capital Improvements Proceeds from sale of real estate and real estate partnership interests Proceeds from Sale of Real Estate Held-for-investment Proceeds from partnership formation Payments for (Proceeds from) Real Estate Partnership Investment, Net Investment in real estate partnerships Payments to Acquire Interest in Subsidiaries and Affiliates Distribution from real estate partnership in excess of earnings Proceeds from Distributions Received from Real Estate Partnerships Leasing costs Increase (Decrease) in Lease Acquisition Costs (Issuance) repayment of mortgage and other notes receivable, net Issuance (Repayments) Of Mortgage And Other Notes Receivable Net The cash outflow from the issuance and purchase of receivables arising from the mortgage note on real estate and other notes receivable, net of repayments. Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities FINANCING ACTIVITIES Net Cash Provided by (Used in) Financing Activities [Abstract] Net (repayment) borrowings under revolving credit facility, net of costs Proceeds from (Repayments of) Lines of Credit Issuance of senior notes, net of costs Proceeds from Issuance of Senior Long-term Debt Redemption and retirement of senior notes Repayments of Senior Debt Repayment of mortgages, capital leases, and notes payable Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities Issuance of common shares, net of costs Proceeds from Issuance of Common Stock Issuance of preferred shares, net of costs Proceeds from Issuance of Preferred Stock and Preference Stock Dividends paid to common and preferred shareholders Dividends Paid To Common And Preferred Shareholders The cash outflow from distribution of an entity's earnings to common and preferred stockholders. Shares withheld for employee taxes Payments Related to Tax Withholding for Share-based Compensation Proceeds from Noncontrolling Interests Proceeds from Noncontrolling Interests Distributions to and redemptions of noncontrolling interests PaymentsOfDividendsAndRedemptionsMinorityInterest The cash outflow for the return on capital for noncontrolled interest in the entity and cash outflow during the period for redemptions of noncontrolling interests. Net cash (used in) provided by financing activities Net Cash Provided by (Used in) Financing Activities Increase (decrease) in cash, cash equivalents, and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Cash, cash equivalents, and restricted cash at beginning of year Cash, cash equivalents, and restricted cash at end of year Stockholders' Equity Note [Abstract] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Preferred Units by Name [Axis] Preferred Units by Name [Axis] Preferred Units, Class [Domain] Preferred Units, Class [Domain] Depositary Shares Depository Shares [Member] Depository Shares 5.0% Series C Cumulative Redeemable Preferred Shares 5.0% Series C Cumulative Redeemable Preferred [Member] Five point Zero Percentage Series C Redeemable Convertible Preferred Stock [Member] 5.417% Series 1 Cumulative Convertible Preferred Shares 5.417% Series 1 Cumulative Convertible Preferred [Member] Five point Four One Seven Percentage Series One Cumulative Convertible Preferred Stock [Member] Subsidiary, Sale of Stock [Axis] Sale of Stock [Axis] Sale of Stock, Name of Transaction [Domain] Sale of Stock [Domain] At The Market Equity Program At The Market Equity Program [Member] At The Market Equity Program [Member] Stock Issuance Costs by Type [Axis] Stock Issuance Costs by Type [Axis] Stock Issuance Costs by Type [Axis] Stock Issuance Costs by Type [Domain] Stock Issuance Costs by Type [Domain] [Domain] for Stock Issuance Costs by Type [Axis] Commissions Commissions [Member] Commissions [Member] Other Offering Costs Other Offering Costs [Member] Other Offering Costs [Member] Class of Stock [Line Items] Class of Stock [Line Items] Preferred shares, shares outstanding Preferred Stock, Shares Outstanding Preferred shares, percentage Preferred Stock, Dividend Rate, Percentage Preferred shares, liquidation preference per share Preferred Stock, Liquidation Preference Per Share Preferred shares, par value Preferred Stock, Par or Stated Value Per Share Conversion price per share Convertible Preferred Stock, Conversion Price Per Share Convertible Preferred Stock, Conversion Price Per Share Shares issued Net cash proceeds of common stock Aggregate offering price of common shares Aggregate Offering Price Of Common Share Maximum aggregate offering price of common shares. Weighted average price per common share Weighted Average Price Per Common Share Weighted average price per common share Commissions related to sales of common shares Payments of Stock Issuance Costs Remaining capacity to issue Remaining Capacity To Issue Remaining $ Capacity Of Shares To Issue Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Restricted Stock [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Unvested at the beginning of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Unvested at the end of year (in shares) Unvested at the beginning of year, Weighted-Average Grant-Date Fair Value (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Granted, Weighted-Average Grant-Date Fair Value (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Vested, Weighted-Average Grant-Date Fair Value (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Forfeited, Weighted-Average Grant-Date Fair Value (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Unvested at the end of year, Weighted-Average Grant-Date Fair Value (in dollars per share) Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Options Employee Stock Option [Member] Range [Axis] Range [Domain] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Vesting Term Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Building and Building Improvements Building and Building Improvements [Member] Minor Improvements, Furniture and Equipment Minor Improvements, Furniture and Equipment [Member] Minor Improvements, Furniture and Equipment [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Estimated useful lives Property, Plant and Equipment, Useful Life Real estate depreciation expense Depreciation Expense Real Estate Depreciation Expense on Real Estate only SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Significant Accounting Policies [Text Block] Minimum annual payments Lessee, Operating Lease, Disclosure [Table Text Block] SHAREHOLDERS’ EQUITY Stockholders' Equity Note Disclosure [Text Block] Income Statement [Abstract] REVENUE Revenues [Abstract] Rental Income Operating Leases, Income Statement, Lease Revenue Other property income Other Revenue (Expense) from Real Estate Operations Mortgage interest income Revenue Mortgage Interest Interest income earned from mortgage notes receivable. Total revenue Revenues EXPENSES Operating Expenses [Abstract] Rental expenses Direct Costs of Leased and Rented Property or Equipment Real estate taxes Real Estate Tax Expense General and administrative General and Administrative Expense Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Total operating expenses Operating Expenses OPERATING INCOME Operating Income (Loss) Other interest income Interest Income, Other Interest expense Interest Expense (Loss) income from real estate partnerships INCOME FROM CONTINUING OPERATIONS Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest Gain on sale of real estate and change in control of interests, net Net income Net income attributable to noncontrolling interests Net Income (Loss) Attributable to Noncontrolling Interest NET INCOME ATTRIBUTABLE TO THE TRUST Net Income (Loss) Attributable to Parent Dividends on preferred shares Preferred Stock Dividends, Income Statement Impact NET INCOME AVAILABLE FOR COMMON SHAREHOLDERS Net Income (Loss) Available to Common Stockholders, Basic EARNINGS PER COMMON SHARE, BASIC Earnings Per Share, Basic [Abstract] Net income available for common shareholders Earnings Per Share, Basic Weighted average common shares outstanding, basic Weighted Average Number of Shares Outstanding, Basic EARNINGS PER COMMON SHARE, DILUTED Earnings Per Share, Diluted [Abstract] Net income available for common shareholders Earnings Per Share, Diluted Weighted average number of common shares Weighted Average Number of Shares Outstanding, Diluted Comprehensive Income Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] COMPREHENSIVE INCOME Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive income attributable to noncontrolling interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest COMPREHENSIVE INCOME ATTRIBUTABLE TO THE TRUST Comprehensive Income (Loss), Net of Tax, Attributable to Parent Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Equity Method Investee Equity Method Investee [Member] Name of Property [Axis] Assembly Row Hotel Assembly Row Hotel [Member] Assembly Row Hotel Derivative Instrument Derivative Instrument [Axis] Derivative, Name [Domain] Derivative Contract [Domain] Interest Rate Swap Interest Rate Swap [Member] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Measurements, Recurring Fair Value, Measurements, Recurring [Member] Derivatives, Fair Value [Line Items] Derivatives, Fair Value [Line Items] Number of interest rate swap agreements Number of Interest Rate Derivatives Held Residual profit maximum Residual Profit Maximum Residual Profit Maximum received as additional interest on mortgage note receivable Aggregate tax basis for Federal tax purposes SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Federal Income Tax Basis Interest Rate Mortgage Loans on Real Estate, Participation Plus Interest Rate, Interest Rate Mortgage Loans on Real Estate, Participation Plus Interest Rate, Interest Rate Derivative, Fixed Interest Rate Derivative, Fixed Interest Rate Summary of quarterly financial data Quarterly Financial Information [Table Text Block] Real Estate [Abstract] Schedule of Real Estate Properties [Table] Schedule of Real Estate Properties [Table] Real Estate Investments by Property Type [Axis] Real Estate Investments by Property Type [Axis] Real Estate Investments by Property Type [Axis] Real Estate Investments, Property Type [Domain] Real Estate Investments, Property Type [Domain] Real Estate Investments, Property Type [Domain] Retail and mixed-use properties Retail And Mixed-Use Properties [Member] Retail And Mixed-Use Properties [Member] Retail properties under capital leases Retail Properties Under Capital Leases [Member] Retail Properties Under Capital Leases [Member] Residential Real Estate Properties [Line Items] Real Estate Properties [Line Items] Cost Real Estate Investment Property, at Cost Accumulated Depreciation and Amortization Real Estate Investment Property, Accumulated Depreciation Encumbrances SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Amount of Encumbrances Fair Value Disclosures [Abstract] Summary of carrying amount and fair value of financial instruments Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block] Summary of financial assets/(liabilities) measured at fair value on a recurring basis Fair Value, Liabilities Measured on Recurring Basis [Table Text Block] Document And Entity Information [Abstract] Document and Entity Information Abstract Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Document Type Document Type Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Trading Symbol Trading Symbol Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity Public Float Entity Public Float Schedule of Business Acquisitions and Dispositions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Equity method investments Equity Method Investments [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Primestor Primestor [Member] Primestor [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Mortgages Mortgages [Member] Hotel Hotel [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] FRIT/Primestor JV FRIT/Primestor JV [Member] FRIT/Primestor JV Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Other Assets Other Assets [Member] Other Liabilities Other Liabilities [Member] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Intangible lease assets acquired Leases, Acquired-in-Place [Member] Intangible lease liabilities acquired Lease liabilities [Member] Lease Liabilities [Member] Above Market Leases Above Market Leases [Member] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Jordan Downs Plaza Jordan Downs Plaza [Member] Jordan Downs Plaza [Member] A building adjacent to Bell Gardens A building adjacent to Bell Gardens [Member] A building adjacent to Bell Gardens Hastings Ranch Plaza Hastings Ranch Plaza [Member] Hastings Ranch Plaza Riverpoint Center Riverpoint Center [Member] Riverpoint Center [Member] Fourth Street 1700-1730 4th Street [Member] 1700-1730 4th Street [Member] Azalea Azalea [Member] Azalea [Member] Bell Gardens Bell Gardens [Member] Bell Gardens [Member] La Alameda La Alameda [Member] La Alameda [Member] Olivo at Mission Hills Olivo at Mission Hills [Member] Olivo at Mission Hills [Member] Plaza Del Sol Plaza Del Sol [Member] Plaza Del Sol [Member] Plaza Pacoima Plaza Pacoima [Member] Plaza Pacoima [Member] Sylmar Towne Center Sylmar Towne Center [Member] Sylmar Town Center 150 Post Street 150 Post Street [Member] 150 Post Street [Member] North Lake Commons North Lake Commons [Member] North Lake Commons [Member] A parcel of land at Bethesda Row Business Acquisition and Disposition Business Acquisition [Line Items] Square footage of real estate property Area of Real Estate Property Construction pre-funding investment Construction pre-funding investment Investment to pre-fund construction due to funding requirement. Controlling interest percentage Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Land parcel acre on which the shopping center will be constructed Area of Land Contribution from third party bank Contribution from third party bank Contribution from third party bank Strike price of put option Strike price of put option Strike price of put option Direct and incremental costs of structuring the NMTC transaction Direct and incremental costs of structuring the NMTC transaction Direct and incremental costs of structuring the NMTC transaction Cash in escrow to be used for development Restricted Cash Contribution of Property Contribution of Property Our investment in joint venture Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures Proceeds from sale Proceeds from Sale of Real Estate Purchase price of real estate acquisition Purchase price of real estate acquisition Purchase price of real estate acquisition Number of condominium units sold Number of condominium units sold Number of condominium units sold Taxes related to the sale of condominiums Gain (Loss) on Sale of Properties, Applicable Income Taxes Assets held for sale Real Estate Held-for-sale Net assets acquired allocated to intangible lease assets or liabilities Finite-lived Intangible Assets Acquired Net assets acquired allocated to below market leases Below Market Lease, Acquired Gross value of acquisition Real estate acquisition, gross value Real estate acquisition, gross value Noncontrolling interest allocated in acquisition Noncontrolling Interest in Variable Interest Entity Number of real estate properties Number of Real Estate Properties Mortgage loans assumed with acquisition Noncash or Part Noncash Acquisition, Debt Assumed Ownership percentage in partnership Equity Method Investment, Ownership Percentage Purchase price of equity method investment Purchase price of equity method investment Purchase price of equity method investment Mortgage loan on unconsolidated real estate property Mortgage Loan on Unconsolidated Real Estate Property Mortgage Loan on Unconsolidated Real Estate Property Our share of unconsolidated mortgage debt Our share of unconsolidated mortgage debt Our share of unconsolidated mortgage debt Gain on sale of real estate, net of income taxes Accounts receivable Accounts Receivable, Gross Asset Acquisitions, Pro Forma depreciation and amortization Asset Acquisitions, Pro Forma depreciation and amortization Asset Acquisitions, Pro Forma depreciation and amortization Asset Acquisitions Pro Forma Revenue Asset Acquisitions Pro Forma Revenue Pro forma revenue as if asset acquisitions happened at the beginning of the period. Asset Acquisitions, Pro Forma Net Inome (Loss) Asset Acquisitions, Pro Forma Net Inome (Loss) Pro Forma net inome (loss) as if the acquisitions had occured at the beginning of the period. Grants of common shares and options Stock Compensation Plan [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Performance Incentive Plan 2010 Performance Incentive Plan 2010 [Member] Performance Incentive Plan 2010 [Member] Number of share-based compensation plans Number Of Share Based Compensation Plans Number Of Share Based Compensation Plans with outstanding grants Number of shares authorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Contractual term (in years) Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Number of options granted in period Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures Weighted average grant date fair value of options granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Total cash received from options exercised Proceeds from Stock Options Exercised Total intrinsic value of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Weighted-average grant-date fair value of stock awarded Total vesting-date fair value of shares vested during the year Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Total unrecognized compensation cost related to unvested share-based compensation arrangements Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Period for recognition (in years) Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition Years Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition Years Weighted-average period for recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Statement of Financial Position [Abstract] 5.0% Series C Cumulative Redeemable Preferred Shares, (stated at liquidation preference $25,000 per share), 6,000 shares issued and outstanding 5.417% Series 1 Cumulative Convertible Preferred Shares, (stated at liquidation preference $25 per share), 399,896 shares issued and outstanding ASSETS Assets [Abstract] Operating (including $1,701,804 and $1,639,486 of consolidated variable interest entities, respectively) Rental Properties Construction-in-progress (including $51,313 and $43,393 of consolidated variable interest entities, respectively) Construction in Progress, Gross Real estate, at cost, total Less accumulated depreciation and amortization (including $292,374 and $247,410 of consolidated variable interest entities, respectively) Net real estate Real Estate Investment Property, Net Accounts and notes receivable Accounts and Notes Receivable, Net Mortgage notes receivable, net Mortgage Loans on Real Estate, Commercial and Consumer, Net Investment in real estate partnerships Prepaid expenses and other assets Prepaid Expense and Other Assets TOTAL ASSETS Assets Liabilities Liabilities [Abstract] Mortgages payable (including $444,388 and $460,372 of consolidated variable interest entities, respectively) Secured Debt Capital lease obligations Capital Lease Obligations Notes payable Notes Payable Senior notes and debentures Senior Notes Accounts payable and accrued expenses Accounts Payable and Accrued Liabilities Dividends payable Dividends Payable Security deposits payable Security Deposit Liability Other liabilities and deferred credits Deferred Credits and Other Liabilities Total liabilities Liabilities Commitments and contingencies (Note 7) Commitments and Contingencies Redeemable noncontrolling interests Redeemable Noncontrolling Interest, Equity, Carrying Amount Shareholders’ equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Preferred shares, authorized 15,000,000 shares, $.01 par: Preferred Stock, Value, Issued Common shares of beneficial interest, $.01 par, 100,000,000 shares authorized, 74,249,633 and 73,090,877 shares issued and outstanding, respectively Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital, Common Stock Accumulated dividends in excess of net income Accumulated Distributions in Excess of Net Income Accumulated other comprehensive (loss) income Accumulated Other Comprehensive Income (Loss), Net of Tax Total shareholders’ equity of the Trust Stockholders' Equity Attributable to Parent Noncontrolling interests Stockholders' Equity Attributable to Noncontrolling Interest Total shareholders’ equity TOTAL LIABILITIES AND SHAREHOLDERS’ EQUITY Liabilities and Equity Earnings Per Share [Abstract] Earnings Per Share Narrative [Table] Earnings Per Share Narrative [Table] Earnings Per Share Narrative [Table] Earnings Per Share Narrative [Line Items] Earnings Per Share Narrative [Line Items] Earnings Per Share Narrative [Line Items] Weighted average unvested shares outstanding Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights Anti-dilutive stock options excluded from dilutive earnings per share calculation Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Cumulative convertible preferred share, dividend rate Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Rollingwood Apartments Rollingwood Apartments [Member] Rollingwood Apartments [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Revolving credit facility Revolving Credit Facility [Member] Mortgages Payable Secured Debt [Member] Notes Payable Notes Payable, Other Payables [Member] Senior Notes and Debentures Senior Notes [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Principal balance Long-term Debt, Gross 2019 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 2020 Long-term Debt, Maturities, Repayments of Principal in Year Two 2021 Long-term Debt, Maturities, Repayments of Principal in Year Three 2022 Long-term Debt, Maturities, Repayments of Principal in Year Four 2023 Long-term Debt, Maturities, Repayments of Principal in Year Five Thereafter Long-term Debt, Maturities, Repayments of Principal after Year Five Long-term Debt Long-term Debt Repayments of mortgage loan Repayments of Secured Debt Revolving credit facility Long-term Line of Credit Summary of share-based compensation expense Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block] Summary of weighted average assumptions used to value options granted Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Summary of option activity Share-based Compensation, Stock Options, Activity [Table Text Block] Summary of restricted share activity Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] Schedule of common shares awarded under various compensation plans Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block] Nature Of Operations [Abstract] Nature Of Operations [Abstract] BUSINESS AND ORGANIZATION Nature of Operations [Text Block] Schedule of principal components of rental income Schedule of Component of Rental Income [Table Text Block] Schedule of Component of Rental Income [Table Text Block] Schedule of minimum rents Schedule Of Minimum Rents Components [Table TextBlock] Schedule of Minimum Rents Components [Table Text Block] Schedule of principal components of rental expenses Schedule Of Principal Components Of Rental Expenses [Table Text Block] Schedule Of Principal Components Of Rental Expenses [Table Text Block] Dividends [Abstract] Common shares, declared Common Stock, Dividends, Per Share, Declared Common shares, paid Common Stock, Dividends, Per Share, Cash Paid Preferred shares, declared Preferred Stock, Dividends Per Share, Declared Preferred shares, paid Preferred Stock, Dividends, Per Share, Cash Paid Payments of ordinary dividends, common stock Common Stock, Ordinary Dividends, Per Share, Cash Paid Common Stock, Ordinary Dividends, Per Share, Cash Paid Payment of ordinary dividends, preferred stock Preferred Stock, Ordinary Dividends, Per Share, Cash Paid Preferred Stock, Ordinary Dividends, Per Share, Cash Paid Common shares, ordinary dividend eligible for 15% rate Common Stock, Ordinary Dividends Eligible For 15 Percent Tax Rate, Per Share, Cash Paid Common Stock, Ordinary Dividends Eligible For 15 Percent Tax Rate, Per Share, Cash Paid Preferred shares, ordinary dividend eligible for 15% rate Preferred Stock, Ordinary Dividends Eligible For 15 Percent Tax Rate, Per Share, Cash Paid Preferred Stock, Ordinary Dividends Eligible For 15 Percent Tax Rate, Per Share, Cash Paid Common shares, qualified dividends, tax rate Common shares, qualified dividends, tax rate The tax rate on qualified dividends for common stock. Preferred stock, qualified dividends, tax rate Preferred Stock, Qualified Dividends, Tax Rate The tax rate for qualified dividends on preferred shares. Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Accumulated Other Comprehensive Income (loss) Term loan TermLoan [Member] TermLoan [Member] Prepaid assets and other expenses prepaid expenses and other assets [Member] prepaid expenses and other assets Fair Value, Balance Sheet Grouping, Financial Statement Captions Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Notional amount of interest rate swap agreements Derivative Liability, Notional Amount Term loan Other Notes Payable Interest rate of variable portion of term loan Fair value of interest rate swaps (less than for 2017) Derivative Liability Other comprehensive income (loss) - change in value of interest rate swaps Interest expense reclassified from other comprehensive loss Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net Noncontrolling Interest in Variable Interest Entity Payments to Noncontrolling Interests Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] 2019 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2020 Operating Leases, Future Minimum Payments, Due in Two Years 2021 Operating Leases, Future Minimum Payments, Due in Three Years 2022 Operating Leases, Future Minimum Payments, Due in Four Years 2023 Operating Leases, Future Minimum Payments, Due in Five Years Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total Operating Leases, Future Minimum Payments Due Subsequent Event [Table] Subsequent Event [Table] Subsequent Event [Line Items] Subsequent Event [Line Items] Income from continuing operations Less: Preferred share dividends Less: Income from continuing operations attributable to noncontrolling interests Net income attributable to noncontrolling interests, net of amounts from gain on real estate Net income attributable to noncontrolling interests, net of amounts from gain on real estate Less: Earnings allocated to unvested shares Earnings Allocated To Unvested Shares Earnings allocated to unvested shares Income from continuing operations available for common shareholders Income Loss From Continuing Operations Available For Common Shareholders This element represents the income or loss from continuing operations attributable to the parent after earnings allocated to unvested shares and adjustments for dividends on preferred stock. Gain on sale of real estate and change in control of interests, net Gain (loss) on change in control of interests and sale of real estate, net Gain (loss) on change in control of interests and sale of real estate, net of noncontrolling interests Net income available for common shareholders, basic and diluted Net income available for common shareholders, basic and diluted Net income after adjustments for dividends on preferred stock and earnings allocated to unvested shares. Weighted average common shares outstanding—basic Stock options Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Weighted average common shares outstanding—diluted Earnings Per Common Share, Basic Earnings Per Common Share, Diluted Income from continuing operations attributable to the Trust Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent ACQUIRED IN-PLACE LEASES Intangible Assets Disclosure [Text Block] SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) Quarterly Financial Information [Text Block] Leases [Abstract] OPERATING LEASES Lessor, Operating Leases [Text Block] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Finite-Lived Intangible Liabilities By Major Class [Axis] Finite-Lived Intangible Liabilities By Major Class [Axis] Finite-Lived Intangible Liabilities By Major Class [Axis] Finite-Lived Intangible Liabilities By Major Class [Domain] Finite-Lived Intangible Liabilities By Major Class [Domain] [Domain] for Finite-Lived Intangible Liabilities By Major Class [Axis] Below Market Leases, Lessor BelowMarketLeasesLessor [Member] Below Market Leases, Lessor Above Market Leases, Lessee Above Market Leases, Lessee [Member] Above Market Leases, Lessee [Member] Leases, Acquired-in-Place Above Market Leases, Lessor AboveMarketLeasesLessor [Member] Above Market Leases, Lessor Below Market Leases, Lessee BelowMarketLeasesLessee [Member] Below Market Leases, Lessee Below Market Lease Below Market Lease [Member] Below Market Lease [Member] Acquired Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets, Net Finite-Lived Intangible Assets, Net Finite-Lived Intangible Assets, Gross Finite-Lived Intangible Assets, Gross Finite-Lived Intangible Assets, Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Finite-Lived Intangible Liabilities, Gross Finite-Lived Intangible Liabilities, Gross Finite-Lived Intangible Liabilities, Gross Finite-Lived Intangible LIabilities, Net Finite-Lived Intangible LIabilities, Net Finite-Lived Intangible LIabilities, Net Finite-Lived Intangible Liabilities, Accumulated Amortization Finite-Lived Intangible Liabilities, Accumulated Amortization Finite-Lived Intangible Liabilities, Accumulated Amortization Amortization of above market leases, lessor Amortization of Intangible Assets Amortization of below market leases, lessor Amortization of Below Market Lease Net increase in rental income Amortization of above and below Market Leases Amortization of Below Market Lease, Lessee Amortization of Below Market Lease, Lessee Amortization of Below Market Lease, Lessee Amortization of above market leases, lessee Amortization Of Intangible Liability Amortization Of Intangible Liability Net increase in rental expense Amortization Of Above And Below Market Leases, Lessee Amortization Of Above And Below Market Leases, Lessee Acquired Finite-lived Intangible Assets, Weighted Average Useful Life Acquired Finite-lived Intangible Assets, Weighted Average Useful Life Acquired Finite-Lived Intangible Liabilities, Weighted Average Useful Life Acquired Finite-Lived Intangible Liabilities, Weighted Average Useful Life Acquired Finite-Lived Intangible Liabilities, Weighted Average Useful Life Acquired Lease Assets Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] 2019 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Acquired Lease Liabilities Below Market Lease, Amortization Income, Maturity Schedule [Abstract] 2019 Below Market Lease, Amortization Income, Next Twelve Months 2020 Below Market Lease, Amortization Income, Year Two 2021 Below Market Lease, Amortization Income, Year Three 2022 Below Market Lease, Amortization Income, Year Four 2023 Below Market Lease, Amortization Income, Year Five Thereafter Below Market Lease, Amortization Income, after Year Five Total future amortization expense from acquired below market leases Below Market Lease, Net Allowance for doubtful accounts Allowance for Doubtful Accounts Receivable Collection of accounts receivable, period Straight Line Rents Collection Of Accounts Receivable Period Straight Line Rents, Collection Of Accounts Receivable, Period Accounts receivable related to straight-line rents Deferred Rent Receivables, Net Property Subject to or Available for Operating Lease, by Major Property Class [Table] Property Subject to or Available for Operating Lease, by Major Property Class [Table] Real estate property Revenue, Major Customer [Line Items] Number of Units in Real Estate Property Number of Units in Real Estate Property Summary of real estate investments and related encumbrances Property, Plant and Equipment [Table Text Block] Signficant property acquisition - Primestor Schedule of Real Estate Properties [Table Text Block] Pro Forma Information, Primestor acquisition Business Acquisition, Pro Forma Information [Table Text Block] 2019 Capital Leases, Future Minimum Payments Due, Next Twelve Months 2020 Capital Leases, Future Minimum Payments Due in Two Years 2021 Capital Leases, Future Minimum Payments Due in Three Years 2022 Capital Leases, Future Minimum Payments Due in Four Years 2023 Capital Leases, Future Minimum Payments Due in Five Years Thereafter Capital Leases, Future Minimum Payments Due Thereafter Future minimum lease payments Capital Leases, Future Minimum Payments Due Less amount representing interest Capital Leases, Future Minimum Payments, Interest Included in Payments Present value Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Level 3 Fair Value, Inputs, Level 3 [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair value of interest rate swaps Principles of Consolidation Consolidation, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Revenue Recognition and Accounts Receivable Revenue Recognition and Accounts Receivable [Policy Text Block] Disclosure of accounting policy for revenue recognition for operating leases and trade and other accounts receivable. Includes how an entity determines the level of its allowance for doubtful accounts. Real Estate Property, Plant and Equipment, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Prepaid Expenses and Other Assets Prepaid Expenses and Other Assets [Policy Text Block] Description of entity's accounting policy related to prepaid expenses and other assets. Debt Issuance Costs Debt, Policy [Policy Text Block] Derivative Instruments Derivatives, Policy [Policy Text Block] Mortgage Notes Receivable Finance, Loan and Lease Receivables, Held-for-investment, Policy [Policy Text Block] Share Based Compensation Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Variable Interest Entities Consolidation, Variable Interest Entity, Policy [Policy Text Block] Redeemable Noncontrolling Interests Redeemable Noncontrolling Interests [Policy Text Block] Redeemable Noncontrolling Interests [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Segment Information Segment Reporting, Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Product and Service [Axis] Product and Service [Domain] Retail and commercial Commercial Real Estate [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Operating Leases, Income Statement, Lease Revenue [Abstract] Operating Leases, Income Statement, Lease Revenue [Abstract] Minimum rents Operating Leases, Income Statement, Minimum Lease Revenue Cost Reimbursement Recovery of Direct Costs Percentage rent Operating Leases, Income Statement, Percentage Revenue Other Other Rental Income This element represents other rental revenue earned during the period. Volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Expected dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected term (in years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Risk free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate FAIR VALUE OF FINANCIAL INSTRUMENTS Fair Value Disclosures [Text Block] EARNINGS PER SHARE Earnings Per Share [Text Block] Real Estate and Accumulated Depreciation, by Property [Table] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, by Property [Table] 29th Place 29th Place [Member] 29th Place [Member] Andorra Andorra [Member] Andorra [Member] Assembly Row/Assembly Square Market Place Bala Cynwyd Bala Cynwyd [Member] Bala Cynwyd [Member] Barcroft Plaza Barcroft Plaza [Member] Barcroft Plaza Barracks Road Barracks Road [Member] Barracks Road [Member] Brick Plaza Brick Plaza [Member] Brick Plaza [Member] Bristol Plaza Bristol [Member] Bristol [Member] Campus Plaza Campus Plaza [Member] Campus Plaza Chelsea Commons CocoWalk CocoWalk [Member] CocoWalk Colorado Blvd Colorado Blvd [Member] Colorado Blvd [Member] Courthouse Center Courthouse Center [Member] Courthouse Center [Member] Crossroads Crossroads [Member] Crossroads [Member] Crow Canyon Commons Crow Canyon [Member] Crow Canyon [Member] Darien Darien [Member] Darien [Member] Dedham Plaza Dedham Plaza [Member] Dedham Plaza [Member] Del Mar Village Del Mar Village [Member] Del Mar Village [Member] East Bay Bridge East Bay Bridge [Member] EastBayBridge [Member] Eastgate Crossing Eastgate [Member] Eastgate [Member] Elllisburg Elllisburg Circle [Member] Elllisburg Circle [Member] Escondido Promenade Escondido Promenade [Member] Former Mervyns Parcel, Escondido Promenade [Member] Falls Plaza Falls Plaza [Member] Falls Plaza [Member] Federal Plaza Federal Plaza [Member] Federal Plaza [Member] Finley Square Finley Square [Member] Finley Square [Member] Flourtown Flourtown [Member] Flourtown [Member] Fourth Street Fourth Street [Member] Fourth Street [Member] Free State Shopping Center Free State Shopping Center [Member] Free State Shopping Center Fresh Meadows Fresh Meadows [Member] Fresh Meadows [Member] Friendship Center Friendship Ctr [Member] Friendship Ctr [Member] Gaithersburg Square Gaithersburg Square [Member] Gaithersburg Square [Member] Garden Market Garden Market [Member] Garden Market [Member] Governor Plaza Governor Plaza [Member] Governor Plaza [Member] Graham Park Plaza Graham Park [Member] Graham Park Gratiot Plaza Gratiot Plaza [Member] Gratiot Plaza [Member] Greenlawn Plaza Greenlawn Plaza [Member] Greenlawn Plaza Greenwich Avenue Greenwich Avenue [Member] Greenwich Avenue [Member] Hastings Ranch Plaza Hauppauge Hauppauge [Member] Hauppauge [Member] Hermosa Avenue Hermosa Ave [Member] Hermosa Ave [Member] Hollywood Blvd. Hollywood Blvd [Member] Hollywood Blvd [Member] Huntington Huntington [Member] Huntington [Member] Huntington Square HuntingtonSquare [Member] Huntington Square [Member] Idylwood Plaza Idylwood Plaza [Member] Idylwood Plaza [Member] Jordan Downs Plaza Kings Court Kings Court [Member] Kings Court [Member] Lancaster Lancaster [Member] Lancaster [Member] Langhorne Square Langhorne Square [Member] Langhorne Square [Member] Laurel Laurel [Member] Laurel [Member] Lawrence Park Lawrence Park [Member] Lawrence Park [Member] Leesburg Plaza Leesburg Plaza [Member] Leesburg Plaza [Member] Linden Square Linden Square [Member] Linden Square [Member] Mount Vernon/South Valley/7770 Richmond Hwy, Mount Vernon [Member] Mount Vernon [Member] North Dartmouth North Dartmouth [Member] North Dartmouth [Member] Northeast Northeast [Member] Northeast [Member] Old Keene Mill Old Keene Mill [Member] Old Keene Mill [Member] Old Town Center Old Town Center [Member] Old Town Center [Member] Pan Am Shopping Center Pan Am Shopping Center [Member] Pan Am Shopping Center [Member] Pentagon Row Pentagon Row [Member] Pentagon Row [Member] Perring Plaza Perring Plaza [Member] Perring Plaza [Member] Pike & Rose Pike & Rose [Member] Pike & Rose Pike 7 Pike 7 [Member] Pike 7 [Member] Plaza Del Mercado Plaza Del Mercado [Member] Plaza Del Mercado Plaza El Segundo/The Point Plaza El Segundo/The Point [Member] Plaza El Segundo/The Point Queen Anne Plaza Queen Anne Plaza [Member] Queen Anne Plaza [Member] Quince Orchard Plaza Quince Orchard Plaza [Member] Quince Orchard Plaza [Member] Rockville Town Square Rockville Town Square [Member] Rockville Town [Member] Rollingwood Apts. Sam's Park and Shop Sams Park and Shop [Member] Sams Park and Shop [Member] San Antonio Center San Antonio Center [Member] San Antonio Center Saugus Saugus [Member] Saugus [Member] The Avenue at White Marsh THE AVENUE at White Marsh [Member] THE AVENUE at White Marsh [Member] The Shoppes At Nottingham Square Shoppes At Nottingham Square [Member] Shoppes At Nottingham Square [Member] The Shops at Sunset Place Sunset Place [Member] Acquisition of the Shops at Sunset Place on 10/1/2015. Third Street Promenade Third Street Promenade [Member] Third Street Promenade [Member] Tower Tower [Member] Tower [Member] Tower Shops Tower Shops January 19 2011 [Member] Tower Shops 1/19/11 [Member] Town Center of New Britain Town Center of New Britain [Member] Town Center of New Britain [Member] Townson Residential Townson Residential [Member] Townson Residential [Member] Troy Troy [Member] Troy [Member] Tyson's Station Tyson Station [Member] Tyson Station [Member] Village at Shirlington Shirlington [Member] Shirlington [Member] Westgate Center Westgate Mall [Member] Westgate Mall [Member] White Marsh Plaza White Marsh Plaza [Member] White Marsh Plaza [Member] White Marsh Other White Marsh Other [Member] White Marsh Other [Member] Wildwood Wildwood [Member] Wildwood [Member] Willow Grove Willow Grove [Member] Willow Grove [Member] Willow Lawn Willow Lawn [Member] Willow Lawn [Member] Wynnewood Wynnewood [Member] Wynnewood [Member] Real Estate and Accumulated Depreciation [Line Items] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items] Initial cost to company, Land SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land Initial cost to company, Buildings and Improvements SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements Cost Capitalized Subsequent to Acquisition SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Cost Capitalized Subsequent to Acquisition, Cost Gross amount at which carried at close of period, Land SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount Gross amount at which carried at close of period, Buildings and Improvements SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Building and Improvements, Amount Gross amount at which carried at close of period, Total SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Gross Accumulated Depreciation and Amortization SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation Life on which depreciation in latest income statements is computed SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Life Used for Depreciation Rollforward of Redeemable Noncontrolling Interests Increase (Decrease) in Temporary Equity [Roll Forward] Beginning balance Net income Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Contributions Redeemable Noncontrolling Interests Contributions Redeemable Noncontrolling Interests Contributions Distributions & Redemptions Redeemable Noncontrolling Interests Distributions Redeemable Noncontrolling Interests Distributions Change in redemption value Temporary Equity, Other Changes Ending balance Redeemable Noncontrolling Interest, by Legal Entity [Table] Redeemable Noncontrolling Interest, by Legal Entity [Table] Business Acquisition [Axis] Redeemable Noncontrolling Interest [Line Items] Redeemable Noncontrolling Interest [Line Items] Payments to Acquire Outstanding Interest Payments to Acquire Outstanding Interest Payments to acquire outstanding interest in partnership Number of downREIT units issued in connection with purchase option Number of downREIT units issued in connection with purchase option Number of downREIT units issued in connection with purchase option Amounts attributable to redeemable noncontrolling interest New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Restatement [Axis] Restatement [Axis] Restatement [Domain] Restatement [Domain] Restatement Adjustment Restatement Adjustment [Member] Previously Reported Previously Reported [Member] Pro Forma Pro Forma [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2016-18 Accounting Standards Update 2016-18 [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Additional cumulative effect Additional cumulative effect [Member] Additional cumulative effect [Member] Condominium sales Condominium sales [Member] Condominium sales New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Percentage of completion gain Percentage of completion gain Percentage of completion gain Cumulative Effect on Retained Earnings, Net of Tax Income Tax Expense (Benefit) Income Tax Expense (Benefit) Cumulative Effect on Retained Earnings, Tax Cumulative Effect on Retained Earnings, Tax Net Cash Provided by (Used in) Operating Activities Net Cash Provided by (Used in) Investing Activities Number of Properties Subject to Ground Leases Number of Properties Subject to Ground Leases Capital Leased Assets, Number of Units Capital Leased Assets, Number of Units Capitalized internal leasing and external legal leasing costs Capitalized internal leasing and external legal leasing costs Internal leasing and external leasing costs that have been capitalized during the period The obligation liability and right of use asset Operating Lease, Right-of-Use Asset Real estate taxes paid by tenants to taxing authorities Realestatetaxespaidbytenants Real estate taxes paid by tenants directly to taxing authorities on our behalf. Property Subject to or Available for Operating Lease [Line Items] Number of states in which entity has properties Number of States in which Entity Operates Number of leases Number Of Leases Number Of Leases Percent of annualized base rent Percent of Annualized Base Rent Maximum Percent of Annualized Base Rent Maximum Operating lease, term Operating Lease, Term Operating Lease, Term Operating Leases, Future Minimum Payments Receivable [Abstract] 2019 Operating Leases, Future Minimum Payments Receivable, Current 2020 Operating Leases, Future Minimum Payments Receivable, in Two Years 2021 Operating Leases, Future Minimum Payments Receivable, in Three Years 2022 Operating Leases, Future Minimum Payments Receivable, in Four Years 2023 Operating Leases, Future Minimum Payments Receivable, in Five Years Thereafter Operating Leases, Future Minimum Payments Receivable, Thereafter Total Operating Leases, Future Minimum Payments Receivable SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Abstract] Schedule IV - Mortgage Loans on Real Estate SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Text Block] Savings and Retirement Plans [Abstract] Savings and Retirement Plans [Abstract] Schedule of Deferred Compensation Arrangement with Individual, Postretirement Benefits [Table] Other Postretirement Benefits, Individual Contracts, Type of Deferred Compensation [Axis] Other Postretirement Benefits, Individual Contracts, Type of Deferred Compensation [Domain] Other Postretirement Benefits, Individual Contracts, Type of Deferred Compensation [Domain] 401(K) and Deferred Compensation Plan Deferred Compensation Arrangement with Individual, by Type of Compensation, Pension and Other Postretirement Benefits [Member] Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items] 401(k) Maximum annual contribution per employee Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount 401(k) Employer match Deferred Compensation Arrangement with Individual, Employer Contribution Per Employee, Matching Percent Deferred Compensation Arrangement with Individual, Employer Contribution Per Employee, Matching Percent 401(k) Maximum of employer match as a percentage of employee eligible earnings Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 401(k) Employer contribution expense Deferred Compensation Arrangement with Individual, Contributions by Employer Deferred compensation liability to participants Deferred Compensation Arrangement with Individual, Recorded Liability SAVINGS AND RETIREMENT PLANS Pension and Other Postretirement Benefits Disclosure [Text Block] Debt Instruments [Abstract] Summary of total debt outstanding Schedule of Debt [Table Text Block] Schedule of principal payments on mortgages payable, notes payable, senior notes and debentures Schedule of Maturities of Long-term Debt [Table Text Block] Schedule of future minimum lease payments and their present value for property under capital leases Schedule of Future Minimum Lease Payments for Capital Leases [Table Text Block] Mortgages payable and notes payable Mortgages Payable And Notes Payable [Member] Mortgages Payable And Notes Payable [Member] Senior notes and debentures Senior Notes And Debentures [Member] Senior Notes And Debentures [Member] Measurement Basis [Axis] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Carrying Value Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Fair value of long-term debt Long-term Debt, Fair Value Extension Option [Axis] Extension Option [Axis] Extension Option [Axis] Extension Option [Domain] Extension Option [Domain] Extension Option [Domain] Extended maturity date by one year Extended maturity date by one year [Member] Loan modificantion for an extension of the term of the loan in which it must be paid. The Grove at Shrewsbury (West) The Grove at Shrewsbury (West) [Member] The Grove at Shrewsbury (West) Term Loan Other Notes Payable Maximum outstanding under revolving credit facility Line of Credit Facility, Maximum Borrowing Capacity Maximum amount of borrowings outstanding Line of Credit Facility, Maximum Amount Outstanding During Period Weighted average amount of borrowings outstanding Line of Credit Facility, Average Outstanding Amount Weighted average interest rate before amortization of debt fees LineofCreditFacilityWeightedAverageInterestRateBeforeAmortizationOfDebt Fees Line of credit facility weighted average interest rate, before amortization of debt fees. Line of credit facility, annual facility fee Line of credit facility, annual facility fee Line of credit facility, annual facility fee Summary of dividends declared and paid per share Schedule of Dividends Declared and Paid Per Share [Table Text Block] Schedule of Dividends Declared and Paid Per Share [Table Text Block] Summary of income tax status of dividends per share paid Schedule Of Income Tax Status Of Dividends Paid [Table Text Block] Schedule Of Income Tax Status Of Dividends Paid [Table Text Block] COMMITMENTS AND CONTINGENCIES Commitments and Contingencies Disclosure [Text Block] Schedule of minimum future commercial property rentals from noncancelable operating leases Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Unconsolidated variable interest entities Unconsolidated Properties [Member] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Number of consolidated variable interest entities Number of Consolidated Variable Interest Entities Number of consolidated entities that meet the criteria of a variable interest entity in which the Company holds a variable interest under ASU 2015-02. Net real estate assets of consolidated VIEs Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net Mortgage Payable From Consolidated Variable Interest Entities Mortgage Payable From Consolidated Variable Interest Entities Mortgages payable balances from variable interest entities, or VIEs, for the periods presented. Maximum exposure to loss in variable interest entity Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount SHARE-BASED COMPENSATION PLANS Disclosure of Compensation Related Costs, Share-based Payments [Text Block] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate Schedule [Table] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate Schedule [Table] Interest Rate [Axis] Interest Rate [Axis] Interest Rate [Axis] Interest Rate [Domain] Interest Rate [Domain] Interest Rate [Domain] Interest Rate One Interest Rate One [Member] Interest Rate One [Member] Interest Rate Two Interest Rate Two [Member] Interest Rate Two [Member] Mortgage Loans on Real Estate, Description, Type of Property [Axis] Real Estate, Type of Property [Axis] Mortgage Loans on Real Estate, Name, Property Type [Domain] Real Estate [Domain] Mortgage On Retail Buildings in Philadelphia, PA, Loan One Mortgage On Retail Buildings in Philadelphia, PA, Loan One [Member] Mortgage On Retail Buildings in Philadelphia, PA, Loan One [Member] Mortgage On Retail Buildings in Philadelphia, PA, Loan Two Mortgage On Retail Buildings in Philadelphia, PA, Loan Two [Member] Mortgage On Retail Buildings in Philadelphia, PA, Loan Two [Member] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Category [Axis] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Category [Axis] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Category [Domain] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Category [Domain] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] Prior Liens SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Prior Lien, Amount Face Amount of Mortgages SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Face Amount of Mortgages Carrying Amount of Mortgages SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate Principal Amount of Loans Subject to delinquent Principal or Interest SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Principal Amount of Delinquent Loans SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Other Disclosure [Abstract] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Other Disclosure [Abstract] SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward] SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward] Balance at beginning of year Collection and satisfaction of loans SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Collections of Principal Issuance of loans SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, New Mortgage Loan Balance at end of year Repairs and maintenance Cost of Property Repairs and Maintenance Utilities Utilities Operating Expense Management fees and costs Management Fee Expense Payroll Property Payroll Expense Property Payroll Expense related to property management which includes salaries and benefits. Marketing Marketing Expense Insurance Real Estate Insurance Bad debt expense Provision for Doubtful Accounts Ground rent Ground Rent Expense Ground Rent Expense Other operating Other Cost of Operating Revenue Total rental expenses Real Estate and Accumulated Depreciation, Carrying Amount of Land and Buildings and Improvements [Abstract] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Gross [Abstract] Balance at beginning of year Acquisitions SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Other Acquisition Improvements SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Improvements Deduction during period—dispositions and retirements of property Real Estate Dispositions and Retirements Carrying amount of real estate sold, cap ex disposals, and transfers to joint venture during the period. Balance at end of year Aggregate cost basis for Federal tax purposes SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Federal Income Tax Basis Reconciliation of Real Estate Accumulated Depreciation [Roll Forward] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation [Roll Forward] Balance at beginning of year Additions during period—depreciation and amortization expense SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation, Depreciation Expense Deductions during period—dispositions and retirements of property Real Estate Accumulated Depreciation Dispositions and Retirements Real Estate Accumulated Depreciation Dispositions and Retirements Balance at end of year Cash in excess of the FDIC insured limit Cash, Uninsured Amount DEBT Debt Disclosure [Text Block] Share-based Compensation, Allocation and Classification in Financial Statements [Abstract] Grants of common shares and options Allocated Share-based Compensation Expense Capitalized share-based compensation Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Share-based compensation expense Share-based Compensation Schedule of earnings per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Operating Leased Assets [Table] Schedule of Operating Leased Assets [Table] Operating Leased Assets [Line Items] Operating Leased Assets [Line Items] Supplemental Cash Flow Information [Abstract] Total interest costs incurred Interest Costs Incurred Interest capitalized Interest Costs Capitalized Adjustment Interest expense Cash paid for interest, net of amounts capitalized Interest Paid, Excluding Capitalized Interest, Operating Activities Cash paid for income taxes Income Taxes Paid, Net NON-CASH INVESTING AND FINANCING TRANSACTIONS (1): Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Mortgage loans refinanced Refinance of mortgage loan Refinance of mortgage loan DownREIT operating partnership units issued with acquisition ValueofdownREITOperatingPartnershipUnitsIssuedinConnectionwithBusinessAcquisition Value of downREIT Operating Partnership Units Issued in Connection with Business Acquisition DownREIT operating partnership units redeemed for common shares Redemptionofoperatingpartnershipunitsforcommonshares Redemption of operating partnership units for common shares Shares issued under dividend reinvestment plan Dividends Reinvested in Noncash Financing Transaction Dividends reinvested in noncash financing transaction Contribution of noncontrolling interest Noncash contribution of noncontrolling interest Noncash contribution of noncontrolling interest Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Outstanding at the beginning of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted (in shares) Exercised (in shares) Forfeited or expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Outstanding at the end of year (in shares) Exercisable at the end of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Outstanding at the beginning of year, Weighted-Average Exercise Price (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Granted, Weighted-Average Exercise Price (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Exercised, Weighted-Average Exercise Price (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Forfeited or Expired, Weighted-Average Exercise Price (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Outstanding at the end of year, Weighted-Average Exercise Price (in dollars per share) Exercisable at the end of the year, Weighted-Average Exercise Price (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Outstanding at the end of year, Weighted-Average Remaining Contractual Term (in years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Exercisable at the end of year, Weighted-Average Remaining Contractual Term (in years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Outstanding at the end of year, Aggregate Intrinsic Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Exercisable at the end of year, Aggregate Intrinsic Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Straight-line rents Straight Line Rent Net amortization of above and below market leases Subsequent Event Subsequent Events [Text Block] THE AVENUE at White Marsh THE AVENUE at White Marsh, Due 2021 [Member] THE AVENUE at White Marsh, Due 2021 [Member] The Grove at Shrewsbury (East) Grove at Shrewsbury (East) [Member] Grove at Shrewsbury (East) [Member] Revolving credit facility Various Various Notes Payable [Member] Various Notes Payable [Member] 2.55% notes 2.55% Senior Notes [Member] 2.55% Senior Notes [Member] 3.00% notes Senior300PercentNotes [Member] Senior300PercentNotes [Member] 2.75% notes Senior 2 Point 75 Percent Notes [Member] Senior 2 Point 75 Percent Notes [Member] 3.95% notes Senior395percentnotes [Member] Senior395percentnotes [Member] 7.48% debentures Debentures 7.48 Percent [Member] Debentures 7.48 Percent [Member] 3.25% Senior notes 3.25% Senior notes [Member] 3.25% Senior notes [Member] 6.82% medium term notes Medium Term Notes 6.82 Percent [Member] Medium Term Notes 6.82 Percent [Member] 4.50% notes 4.50% Senior notes [Member] 4.50% Senior notes 3.625% Senior notes 3.625% Senior notes [Member] 3.625% Senior notes [Member] Mortgages Payable Debt Instrument, Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate Net unamortized (discount)/premium and debt issuance costs Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net Interest rate Debt Instrument, Interest Rate, Stated Percentage Mortgages payable Net unamortized debt issuance costs Debt Issuance Costs, Net Total debt and capital lease obligations Debt and Capital Lease Obligations Rollforward table of the redeemable noncontrolling interest Redeemable Noncontrolling Interest [Table Text Block] Schedule of supplemental disclosures related to the Consolidated Statements Of Cash Flows Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Reconcilition of cash, cash equivalents, and restricted cash Schedule of Cash and Cash Equivalents [Table Text Block] DIVIDENDS Dividends Disclosure [Text Block] This text block may be used to disclose all or some of the information related to dividends declared, dividends paid, and the tax status of dividends as of the financial reporting date. Operating real estate, consolidated variable interest entities Operating Real Estate From Consolidated Variable Interest Entities Operating real estate balances from variable interest entities, or VIEs, for the periods presented. Construction-in-progress, consolidated variable interest entities Constructioninprogresofconsolidatedvariableieestentity Construction-in-progress oc consolidated variable interest entity Accumulated depreciation and amortization, consolidated variable interest entities Accumulated depreciation and amortization, consolidated variable interest entities Accumulated depreciation and amortization related to real estate, Consolidated Variable Interest Entities. Mortgages payable, consolidated variable interest entities Preferred shares, authorized Preferred Stock, Shares Authorized Preferred shares, par value Preferred Shares, liquidation preference per share Preferred shares, shares issued Preferred Stock, Shares Issued Common shares of beneficial interest, par value Common Stock, Par or Stated Value Per Share Common shares of beneficial interest, shares authorized Common Stock, Shares Authorized Common shares of beneficial interest, shares issued Common Stock, Shares, Issued Common shares of beneficial interest, shares outstanding Common Stock, Shares, Outstanding REAL ESTATE Real Estate Acquisitions Dispositions Text Block Disclosure of real estate acquisitions and dispositions, and at year end, a summary of our real estate investments and related encumbrances. Revenue Operating income NET INCOME ATTRIBUTABLE TO THE TRUST Net income available for common shareholders