Schedule III - Real Estate and Accumulated Depreciation and Amortization - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate and Accumulated Depreciation [Line Items]
Total cost basis for federal income tax purposes	$ 4,700,000
Reconciliation of real estate owned:
Balance at the beginning of year	3,533,172	$ 3,789,711	$ 3,671,560
Additions during year	676,355	291,004	478,717
Properties sold and impaired during the year	(270,241)	(527,597)	(343,976)
Other reclassifications	(2,827)	(19,946)	(16,590)
Balance at end of year	3,936,459	3,533,172	3,789,711
Reconciliation of accumulated depreciation and amortization:
Balance at the beginning of year	844,931	812,207	795,486
Depreciation and amortization expense	139,493	128,384	124,618
Accumulated depreciation and amortization of properties sold, impaired and held for sale during year	(93,455)	(86,428)	(106,268)
Other reclassifications	0	(9,232)	(1,629)
Balance at end of year	890,969	844,931	812,207
LCIF [Member]
Real Estate and Accumulated Depreciation [Line Items]
Total cost basis for federal income tax purposes	900,000
Reconciliation of real estate owned:
Balance at the beginning of year	731,202	1,061,606	910,113
Additions during year	123,261	49,417	152,280
Properties sold and impaired during the year	(60,221)	(379,821)	(787)
Balance at end of year	794,242	731,202	1,061,606
Reconciliation of accumulated depreciation and amortization:
Balance at the beginning of year	182,505	199,690	176,167
Depreciation and amortization expense	30,701	26,989	23,523
Accumulated depreciation and amortization of properties sold, impaired and held for sale during year	(34,830)	(44,174)	0
Balance at end of year	$ 178,376	$ 182,505	$ 199,690