Selected Quarterly Financial Data (Unaudited) (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
Quarterly Financial Data [Abstract]
Rental income	$ 61,685	$ 62,092	$ 62,113	$ 62,659	$ 64,625	$ 64,158	$ 62,428	$ 59,820	$ 248,549	$ 251,031	$ 226,910
Net income (loss)	(2,339)	$ 16,911	$ (191,163)	$ (33,370)	$ 14,114	$ 12,622	$ 14,608	$ 15,190	$ (209,961)	$ 56,534	$ 54,620
Non-cash loss related to distribution of RMR shares	$ 12,368
Per Share data:
Net income (loss) per common share (basic and diluted)	$ (0.03)	$ 0.24	$ (2.71)	$ (0.47)	$ 0.20	$ 0.19	$ 0.27	$ 0.28	$ (2.97)	$ 0.92	$ 1.00
Common distributions declared (in dollars per share)	0.56	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 1.72	$ 1.72	$ 1.72
Non-cash distribution, per share, related to distribution of shares	$ 0.13