SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION - Property Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate Assets
Balance, beginning of period	$ 2,021,494	$ 2,061,372	$ 2,036,794
Additions
Improvements	26,078	31,844	31,342
Property acquisitions	18,770	0	11,041
Deductions:
Asset sales	(815,357)	(61,801)	(6,237)
Impairment	(13,100)	0	0
Retirements	(9,105)	(9,921)	(11,568)
Balance, end of period	1,228,780	2,021,494	2,061,372
Accumulated Depreciation
Balance, beginning of period	(414,552)	(369,661)	(320,857)
Additions: Depreciation	(49,427)	(62,239)	(61,915)
Asset sales	183,819	7,427	1,543
Deductions: retirements	9,105	9,921	11,568
Balance, end of period	$ (271,055)	$ (414,552)	$ (369,661)