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Investment in Unconsolidated Real Estate Ventures (Textual) (Details) ft² in Millions |  |  |  |  | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Apr. 12, 2019 USD ($) | Apr. 11, 2019 USD ($) property | Apr. 01, 2019 USD ($) | Mar. 29, 2019 USD ($) property | Jun. 30, 2019 USD ($) ft² a property Real_Estate_Investment Apartment_unit | Mar. 31, 2019 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) ft² a property Parcel Real_Estate_Investment Apartment_unit | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($)
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Accounts receivable |  |  |  |  | $ 18,181,000 |  |  | $ 18,181,000 |  | $ 16,394,000
Cumulative effect of accounting change |  |  |  |  |  | $ (5,336,000) |  |  |  |
Number of Properties | property |  |  |  |  | 96 |  |  | 96 |  |
Right of Use Asset [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  |  | $ (4,600,000) |  |
Office Properties [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of Properties | property |  |  |  |  | 88 |  |  | 88 |  |
Real Estate Venture [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Accounts receivable |  |  |  |  | $ 400,000 |  |  | $ 400,000 |  | $ 800,000
Real Estate Venture [Member] | Management Fees [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  | 1,100,000 |  | $ 1,300,000 | 2,200,000 | $ 2,600,000 |
Real Estate Venture [Member] | Leasing Commission Income [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  | $ 300,000 |  | $ 1,500,000 | $ 500,000 | $ 2,900,000 |
Unconsolidated Real Estate Ventures [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  | 10 |  |  | 10 |  |
Investment in Real Estate Ventures, equity method |  |  |  |  | $ 128,200,000 |  |  | $ 128,200,000 |  |
Real estate ventures aggregate indebtedness |  |  |  |  | $ 605,500,000 |  |  | $ 605,500,000 |  |
Unconsolidated Real Estate Ventures [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 15.00% |  | 25.00% | 15.00% | 25.00% |
Unconsolidated Real Estate Ventures [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 70.00% |  | 70.00% | 70.00% | 70.00% |
Unconsolidated Real Estate Ventures [Member] | Six Real Estate Ventures [Member] | Office Properties [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  | 6 |  |  | 6 |  |
Rentable Square Feet | ft² |  |  |  |  | 5.8 |  |  | 5.8 |  |
Unconsolidated Real Estate Ventures [Member] | Two Real Estate Ventures [Member] | Land Held For Development [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  | 2 |  |  | 2 |  |
Acreage of Land | a |  |  |  |  | 1.4 |  |  | 1.4 |  |
Unconsolidated Real Estate Ventures [Member] | One Real Estate Venture [Member] | Land Under Active Development [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  | 1 |  |  | 1 |  |
Acreage of Land | a |  |  |  |  | 1.3 |  |  | 1.3 |  |
Unconsolidated Real Estate Ventures [Member] | One Other Real Estate Venture [Member] | Residential Tower [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  | 1 |  |  | 1 |  |
Number of Property units | Apartment_unit |  |  |  |  | 321 |  |  | 321 |  |
MAP Venture [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 50.00% |  | 50.00% | 50.00% | 50.00% |
Number of lessee land parcels | Parcel |  |  |  |  |  |  |  | 58 |  |
Cumulative effect of accounting change |  |  |  |  |  |  |  | $ 9,200,000 |  |
Lessee under ground lease term |  |  |  |  |  |  |  | 99 years |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  | 15.00% |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Indebtedness to third parties, refinanced amount |  | $ 115,300,000 |  | $ 134,100,000 |  |  |  |  |  |
Number of Properties | property |  | 4 |  | 4 |  |  |  |  |  |
Debt instrument, initial advance |  |  | $ 111,000,000.0 | $ 113,100,000 |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 1.80% |  | 1.95% |  |  |  |  |  |
Debt instrument maturity date |  |  |  | Mar. 29, 2024 |  |  |  |  |  |
Debt Instrument, maturity date |  | Apr. 11, 2024 |  |  |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Secured Debt [Member] | Distributions from Loan 1 [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, initial advance | $ 16,700,000 |  |  |  |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Secured Debt [Member] | Distributions from Loan 2 [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, initial advance | $ 13,800,000 |  |  |  |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Minimum [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effective interest rate |  |  |  | 5.45% |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Maximum [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Effective interest rate |  |  |  | 6.45% |  |  |  |  |  |
Debt Instrument, interest rate, stated percentage |  | 6.30% |  |  |  |  |  |  |  |
PJP VII [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Guarantees, maximum exposure amount |  |  |  |  | $ 300,000 |  |  | $ 300,000 |  |
Construction loan, principal amount |  |  |  |  | 3,700,000 |  |  | 3,700,000 |  |
4040 Wilson Venture [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Guarantees, maximum exposure amount |  |  |  |  | 41,300,000 |  |  | 41,300,000 |  |
Construction loan total borrowing capacity |  |  |  |  | $ 150,000,000.0 |  |  | $ 150,000,000.0 |  |

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