Schedule III - Real Estate and Accumulated Depreciation - Reconciliation of Gross Real Estate Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance, beginning of year	$ 13,539,921	$ 14,566,343	$ 15,857,507
Additions:
Acquisitions	634,080	91,052	33,695
Improvements	28,503	25,781	60,321
Deductions/Other:
Dispositions	(505,403)	(878,552)	(1,261,724)
Impairments	(82,292)	(228,750)	(106,064)
Reclassification to assets held for sale	(52,376)	(36,722)	(16,761)
Other	15,267	769	(631)
Balance, end of year	$ 13,577,700	$ 13,539,921	$ 14,566,343