Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 274,296	$ 263,983	$ 261,367	$ 258,375	$ 267,519	$ 255,275	$ 247,402	$ 228,577	$ 1,058,022	$ 998,773	$ 844,887
Net income	$ 42,885	$ 27,903	$ 39,233	$ 31,272	$ 9,544	$ 38,249	$ 36,387	$ 39,789	137,234	123,968	153,184
Per share data (basic and diluted):
Net income (in dollars per share)	$ 0.18	$ 0.12	$ 0.17	$ 0.13	$ 0.04	$ 0.16	$ 0.15	$ 0.18
Common distributions declared (in dollars per share)	$ 0.39	$ 0.39	$ 0.39	$ 0.39	$ 0.52	$ 0.39	$ 0.39	$ 0.39
Loss on distribution to common shareholders of The RMR Group Inc. common stock					$ 38,437				$ 0	$ 38,437	$ 0
Non-cash distribution (in dollars per share)					$ 0.13