Real Estate Investments - Gross Carrying Value of Properties (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment, Gross [Abstract]
Land		$ 470,391	$ 469,522
Tenant improvements		451,807	433,686
Operating properties		3,769,067	3,586,295
Assets held for sale - real estate investments	[1],[2]	47,291	73,591
Total		3,816,358	3,659,886
Building and Improvements [Member]
Property, Plant and Equipment, Gross [Abstract]
Operating properties		$ 2,846,869	$ 2,683,087

[1] 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 sheet of the property 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 sheet of the property held for sale. See “Held for Sale” below in this Note 3.