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Real Estate Investments - Summary of Properties Sold (Parenthetical) (Details) $ in Thousands |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 20, 2018 USD ($) ft² property | Sep. 30, 2018 USD ($) property | Dec. 31, 2018 USD ($) ft² | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |
Area of Real Estate Property | ft² |  |  | 16,777,249 |  |
Net gain on disposition of real estate |  |  | $ 2,932 | $ 31,657 | $ 116,983
Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |
Number of properties contributed to a joint venture | property |  | 8 |  |  |
Impairment on assets held for sale |  | $ 56,900 |  |  |
Rockpoint Portfolio [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |
Number of properties contributed to a joint venture | property | 8 |  |  |  |
Area of Real Estate Property | ft² | 1,293,197 |  |  |  |
Aggregate gross sales price of a joint venture | $ 312,000 |  |  |  |
Impairment on assets held for sale |  | 56,900 |  |  |
Closing costs | 2,200 |  |  |  |
Working capital contributions | 600 |  |  |  |
Cash proceeds at settlement | 262,400 |  |  |  |
Net gain on disposition of real estate | $ 400 |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |
Number of properties contributed to a joint venture | property | 8 |  |  |  |
Area of Real Estate Property | ft² | 1,293,197 |  |  |  |
Aggregate gross sales price of a joint venture | $ 312,000 |  |  |  |
Equity method investment percentage | 15.00% |  |  |  |
Equity method investment percentage of parnter | 85.00% |  |  |  |
Impairment on assets held for sale |  | $ 56,900 |  |  |
Closing costs | $ 2,200 |  |  |  |
Working capital contributions | 600 |  |  |  |
Net gain on disposition of real estate | 400 |  |  |  |
Cash paid for acquisition | $ 265,200 |  |  |  |

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