Schedule III Real Estate Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate and Accumulated Depreciation [Line Items]
Real Estate, Gross	$ 7,305,848	$ 7,031,660	$ 6,708,250
Real Estate, Other Acquisitions	28,025	117,981	474,213
Real Estate, Improvements	421,404	592,651	498,097
Real Estate, Other Additions	0	0	14,081
Real Estate, Cost of Real Estate Sold	(1,468,635)	(350,698)	(591,966)
Impairment Charges	(3,406)	(15,406)	0
Write Off of Fully Depreciated Assets	(101,359)	(70,340)	(71,015)
SEC Schedule III, Real Estate, Gross	6,181,877	7,305,848	7,031,660
Total Land, Land Improvements, Buildings and Tenant Improvements Held-for-Sale	(49,277)	(906,591)	(61,927)
SEC Schedule III, Real Estate, Excluding Held-for-Sale	6,132,600	6,399,257	6,969,733
Real Estate Accumulated Depreciation	1,199,608	1,505,677	1,382,757	$ 1,296,685
Real Estate Accumulated Depreciation, Depreciation Expense	253,683	290,279	288,583
Real Estate Accumulated Depreciation, Real Estate Sold	(458,393)	(97,032)	(131,496)
Real Estate Accumulated Depreciation, Other Deductions	(101,359)	(70,327)	(71,015)
Schedule III - Accumulated Depreciation of Assets Held-for-Sale	(7,183)	(270,340)	(14,351)
Real Estate Investment Property, Accumulated Depreciation	$ 1,192,425	$ 1,235,337	$ 1,368,406