Schedule III-REAL ESTATE AND ACCUMULATED DEPRECIATION - Property Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Real Estate Assets
Balance, beginning of period	$ 2,036,794	$ 1,953,848	$ 1,907,903
Additions
Property acquisitions	11,041	63,528	38,835
Improvements	31,342	28,879	24,520
Asset sales	(6,237)
Deductions: retirements	(11,568)	(9,461)	(17,410)
Balance, end of period	2,061,372	2,036,794	1,953,848
Accumulated Depreciation
Balance, beginning of period	(320,857)	(271,683)	(231,080)
Additions: Depreciation	(61,915)	(58,635)	(58,013)
Asset sales	1,543
Deductions: retirements	11,568	9,461	17,410
Balance, end of period	$ (369,661)	$ (320,857)	$ (271,683)