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.