Schedule III - Real Estate and Accumulated Depreciation and Amortization - Summary (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
Total cost basis for federal income tax purposes	$ 4,400,000,000
Reconciliation of real estate owned:
Balance at the beginning of year	3,812,294,000	3,564,466,000	3,172,246,000
Additions during year	210,143,000	492,437,000	540,847,000
Properties sold during year	(282,143,000)	(212,771,000)	(138,041,000)
Properties impaired during the year	(65,426,000)	(31,741,000)	(10,553,000)
Other reclassifications	(3,308,000)	(97,000)	(33,000)
Balance at end of year	3,671,560,000	3,812,294,000	3,564,466,000
Reconciliation of accumulated depreciation and amortization:
Balance at the beginning of year	775,617,000	738,068,000	638,368,000
Depreciation and amortization expense	119,156,000	122,057,000	119,067,000
Accumulated depreciation and amortization of properties sold, impaired and held for sale during year	(98,698,000)	(84,508,000)	(19,367,000)
Other reclassifications	(589,000)	0	0
Balance at end of year	$ 795,486,000	$ 775,617,000	$ 738,068,000