Real Estate Investments - Gross Carrying Value of Company's Properties (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Property, Plant and Equipment, Gross [Abstract]
Land		$ 492,197	$ 469,522
Tenant improvements		444,038	433,686
Operating properties		3,832,348	3,586,295
Assets held for sale - real estate investments	[1]	0	73,591	[2]	$ 794,588
Total		3,832,348	3,659,886
Building and Improvements [Member]
Property, Plant and Equipment, Gross [Abstract]
Operating properties		$ 2,896,113	$ 2,683,087

[1] As of December 31, 2015, the Och-Ziff Portfolio and Cira Square were held for sale. As of December 31, 2016, the Concord Airport Plaza located in the Other segment and the Calverton portfolio located in the Metropolitan Washington, D.C. segment was held for sale. See Note 3, “Real Estate Investments,” for further information.

[2] Real estate investments related to assets held for sale above represents gross real estate assets and does not include accumulated depreciation, land held for development or other assets on the balance sheets of the properties held for sale. See 2016 Held for Sale section below.