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Real Estate Investments - Gross Carrying Value of Operating Properties (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Property, Plant and Equipment, Gross [Abstract] |  |  |
Land |  | $ 437,640 | $ 492,197
Tenant improvements |  | 411,668 | 444,038
Operating properties |  | 3,429,048 | 3,832,348
Assets held for sale - real estate investments | [1] | 396,072 | 0
Total |  | 3,825,120 | 3,832,348
Building and Improvements [Member] |  |  |
Property, Plant and Equipment, Gross [Abstract] |  |  |
Operating properties |  | $ 2,579,740 | $ 2,896,113
[1] | As of September 30, 2018, eight office properties in the Metropolitan Washington, D.C. segment were classified as held for sale. See Note 3, “Real Estate Investments,” for further information.

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