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Investment in Unconsolidated Real Estate Ventures (Textual) (Details) |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |
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Apr. 12, 2019 USD ($) | Apr. 01, 2019 USD ($) | Mar. 29, 2019 USD ($) property | Jan. 01, 2019 USD ($) | Sep. 30, 2019 USD ($) a ft² apartment investment | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) a ft² parcel apartment investment | Sep. 30, 2018 USD ($) | Aug. 29, 2019 USD ($) ft² | Apr. 11, 2019 USD ($) | Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Accounts receivable |  |  |  |  | $ 13,872,000 |  | $ 13,872,000 |  |  |  | $ 16,394,000
Cumulative effect of accounting change |  |  |  | $ (5,336,000) |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 15.00% |  | 15.00% |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 70.00% |  | 70.00% |  |  |  |
Real Estate Venture |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Accounts receivable |  |  |  |  | $ 800,000 |  | $ 800,000 |  |  |  | $ 800,000
Real Estate Venture | Management Fees |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  | 1,000,000.0 | $ 1,400,000 | 3,200,000 | $ 4,000,000.0 |  |  |
Real Estate Venture | Leasing Commission Income |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  | 300,000 | 1,500,000 | 800,000 | 4,400,000 |  |  |
Unconsolidated Real Estate Ventures |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in Real Estate Ventures, equity method |  |  |  |  | 127,800,000 |  | 127,800,000 |  |  |  |
Gain on early extinguishment of debt |  |  |  |  | 4,371,000 | $ (334,000) | 4,371,000 | $ (1,052,000) |  |  |
Real estate ventures aggregate indebtedness |  |  |  |  | $ 592,400,000 |  | $ 592,400,000 |  |  |  |
Unconsolidated Real Estate Ventures | Minimum |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 15.00% | 25.00% | 15.00% | 25.00% |  |  |
Unconsolidated Real Estate Ventures | Maximum |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 70.00% | 70.00% | 70.00% | 70.00% |  |  |
Unconsolidated Real Estate Ventures | Six Real Estate Ventures | Office properties |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | investment |  |  |  |  | 6 |  | 6 |  |  |  |
Rentable area (in square feet) | ft² |  |  |  |  | 5,600,000 |  | 5,600,000 |  |  |  |
Unconsolidated Real Estate Ventures | Two Real Estate Ventures | Land Held For Development |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | investment |  |  |  |  | 2 |  | 2 |  |  |  |
Area of land (in acres) | a |  |  |  |  | 1.4 |  | 1.4 |  |  |  |
Unconsolidated Real Estate Ventures | One Real Estate Venture | Land Under Active Development |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | investment |  |  |  |  | 1 |  | 1 |  |  |  |
Area of land (in acres) | a |  |  |  |  | 1.3 |  | 1.3 |  |  |  |
Unconsolidated Real Estate Ventures | One Other Real Estate Venture | Residential Tower |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | investment |  |  |  |  | 1 |  | 1 |  |  |  |
Number of Property units | apartment |  |  |  |  | 321 |  | 321 |  |  |  |
Falls Church, Virginia |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Rentable area (in square feet) | ft² |  |  |  |  |  |  |  |  | 180,659 |  |
Mortgage loan on real estate |  |  |  |  |  |  |  |  | $ 26,000,000.0 |  |
Gain on early extinguishment of debt |  |  |  |  | $ 2,200,000 |  |  |  |  |  |
MAP Venture |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 50.00% | 50.00% | 50.00% | 50.00% |  |  |
Gain on early extinguishment of debt |  |  |  |  | $ 0 | $ (334,000) | $ 0 | $ (334,000) |  |  |
Number of lessee land parcels | parcel |  |  |  |  |  |  | 58 |  |  |  |
Lessee under ground lease term |  |  |  |  |  |  | 99 years |  |  |  |
Cumulative effect of accounting change |  |  |  | $ 9,200,000 |  |  | $ 4,600,000 |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  | 15.00% |  |  |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC | Secured Debt |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Indebtedness to third parties, refinanced amount |  |  | $ 134,100,000 |  |  |  |  |  |  |  |
Number of properties used to secure debt | property |  |  | 4 |  |  |  |  |  |  |  |
Debt instrument, initial advance |  | $ 111,000,000.0 | $ 113,100,000 |  |  |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC | Secured Debt | Distributions from Loan 1 |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, initial advance | $ 16,700,000 |  |  |  |  |  |  |  |  |  |
Spread on variable rate |  |  |  |  |  |  | 1.95% |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC | Secured Debt | Distributions from Loan 2 |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Indebtedness to third parties, refinanced amount |  |  |  |  |  |  |  |  |  | $ 115,300,000 |
Debt instrument, initial advance | $ 13,800,000 |  |  |  |  |  |  |  |  |  |
Spread on variable rate |  |  |  |  |  |  | 1.80% |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC | Minimum | Secured Debt | Distributions from Loan 1 |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Effective interest rate |  |  |  |  | 5.45% |  | 5.45% |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC | Maximum | Secured Debt | Distributions from Loan 1 |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Effective interest rate |  |  |  |  | 6.45% |  | 6.45% |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC | Maximum | Secured Debt | Distributions from Loan 2 |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 6.30% |  | 6.30% |  |  |  |
PJP VII |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Construction loan, principal amount |  |  |  |  | $ 3,700,000 |  | $ 3,700,000 |  |  |  |
Guarantees, maximum exposure amount |  |  |  |  | 300,000 |  | 300,000 |  |  |  |
4040 Wilson Venture |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Indebtedness to third parties, refinanced amount |  |  |  |  | 150,000,000.0 |  | 150,000,000.0 |  |  |  |
Guarantees, maximum exposure amount |  |  |  |  | $ 41,300,000 |  | $ 41,300,000 |  |  |  |

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