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Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details) $ in Thousands |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 20, 2018 USD ($) ft² property | Dec. 11, 2018 USD ($) ft² property | Dec. 31, 2018 USD ($) ft² a property | Dec. 31, 2017 USD ($) a | Dec. 31, 2016 USD ($) property | Mar. 31, 2017 property | Dec. 31, 2016 Office | Mar. 31, 2016 property
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Area of Real Estate Property | ft² |  |  | 16,777,249 |  |  |  |  |
Proceeds from the sale of properties |  |  | $ 324,090 | $ 171,860 | $ 784,331 |  |  |
Number of Properties | property |  |  | 97 |  |  |  |  |
Assets Held-for-sale [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Acreage of land | a |  |  |  | 13.1 |  |  |  |
Assets Held-for-sale [Member] | Land Held For Development [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Acreage of land | a |  |  | 37.9 |  |  |  |  |
Metropolitan DC [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Number of Properties |  |  |  |  |  |  | 3 | 2
Other [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Number of Properties |  |  | 1 |  | 3 | 4 | 2 |
Other [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Acreage of land | a |  |  | 35.2 | 13.1 |  |  |  |
Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Acreage of land | a |  |  | 2.7 |  |  |  |  |
DRA Austin Venture [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Area of Real Estate Property | ft² |  | 1,570,123 |  |  |  |  |  |
Ownership percentage |  | 50.00% |  |  |  |  |  |
Number of Properties | property |  | 12 |  |  |  |  |  |
Aggregate purchase price |  | $ 535,100 |  |  |  |  |  |
3001-3003 and 3025 JFK Boulevard [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Lease agreement term |  |  | 99 years |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [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 |  |  |  |  |  |  |
Ownership percentage | 15.00% |  |  |  |  |  |  |
Cash proceeds received from sale of properties | $ 265,200 |  |  |  |  |  |  |
Closing costs | 2,200 |  |  |  |  |  |  |
Working capital contribution | 600 |  |  |  |  |  |  |
Proceeds from the sale of properties | $ 262,400 |  |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member] | Rockpoint [Member] |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Ownership percentage | 85.00% |  |  |  |  |  |  |

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