Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in cost of properties
Beginning balance	$ 3,874,715	$ 4,158,616	$ 4,014,336
Acquisitions of operating properties	0	0	194,616
Improvements and other additions	259,548	251,960	273,761
Sales	(138,216)	(268,038)	(172,628)
Impairments	(15,116)	(143,502)	(29,548)
Other dispositions	(118)	(124,321)	(121,921)
Ending balance	3,980,813	3,874,715	4,158,616
Changes in accumulated depreciation
Beginning balance	715,951	718,680	703,083
Depreciation expense	107,772	105,763	112,695
Sales	(22,567)	(56,607)	(49,614)
Impairments	0	(42,161)	(6,092)
Other dispositions	(118)	(9,724)	(41,392)
Ending balance	$ 801,038	$ 715,951	$ 718,680