Schedule III-Real Estate and Accumulated Depreciation - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in total real estate assets
Balances, beginning of year	$ 9,209,211	$ 10,689,656	$ 12,777,882
Additions	202,280	254,604	392,575
Dispositions and retirements	(283,958)	(1,735,049)	(2,480,801)
Balances, end of year	9,127,533	9,209,211	10,689,656
Aggregate gross cost of the property for federal income tax purposes	8,551,235
Changes in accumulated depreciation
Balances, beginning of year	1,851,901	1,892,744	1,709,992
Additions	277,917	277,270	354,977
Dispositions and retirements	(111,515)	(318,113)	(172,225)
Balances, end of year	$ 2,018,303	$ 1,851,901	$ 1,892,744
Buildings and improvements | Minimum
REAL ESTATE AND ACCUMULATED DEPRECIATION
Estimated useful lives of assets	5 years
Buildings and improvements | Maximum
REAL ESTATE AND ACCUMULATED DEPRECIATION
Estimated useful lives of assets	40 years
Tenant improvements | Minimum
REAL ESTATE AND ACCUMULATED DEPRECIATION
Estimated useful lives of assets	5 years
Tenant improvements | Maximum
REAL ESTATE AND ACCUMULATED DEPRECIATION
Estimated useful lives of assets	7 years
Equipment and furnishings | Minimum
REAL ESTATE AND ACCUMULATED DEPRECIATION
Estimated useful lives of assets	5 years
Equipment and furnishings | Maximum
REAL ESTATE AND ACCUMULATED DEPRECIATION
Estimated useful lives of assets	7 years