Supplementary Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplementary Quarterly Financial Data [Abstract]
Rental income	$ 94,550	$ 93,322	$ 92,948	$ 92,315	$ 91,321	$ 95,627	$ 93,986	$ 95,321	$ 373,135	$ 376,255	$ 359,246
Cost of operations	28,973	30,448	30,057	31,746	29,290	33,102	31,535	33,444	121,224	127,371	114,831
Net income allocable to common shareholders	$ 14,906	$ 22,484	$ 11,129	$ 19,771	$ 82,131	$ 11,268	$ 9,826	$ 9,940	$ 68,291	$ 113,154	$ 43,851
Net income per share: Basic	$ 0.55	$ 0.83	$ 0.41	$ 0.73	$ 3.05	$ 0.42	$ 0.37	$ 0.37	$ 2.53	$ 4.21	$ 1.77
Net income per share: Diluted	$ 0.55	$ 0.83	$ 0.41	$ 0.73	$ 3.04	$ 0.42	$ 0.36	$ 0.37	$ 2.52	$ 4.19	$ 1.77