Real Estate Investments Gross Carrying Value of Company's Properties (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment, Gross [Abstract]
Land		$ 658,280	$ 669,635
Building and improvements		3,438,563	3,409,303
Tenant improvements		532,380	524,754
Real Estate Investments		4,629,223	4,603,692
Assets held for sale - real estate investments	[1]	74,706	27,436
Total		$ 4,703,929	$ 4,631,128

[1] Real estate investments related to assets held for sale above represents gross real estate assets and does not include accumulated depreciation or other assets on the balance sheets of the properties held for sale.