Shareholders' Equity - Distributions (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended								12 Months Ended
	Feb. 22, 2018	Jan. 19, 2018	Dec. 14, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Share Distributions
Cash distributions paid or payable												$ 340,084	$ 314,135	$ 299,967
Distributions per common share (in dollars per share)				$ 0.52	$ 0.52	$ 0.52	$ 0.51	$ 0.51	$ 0.51	$ 0.51	$ 0.50
Common shares
Common Share Distributions
Cash distributions paid or payable (in dollars per share)												$ 2.07	$ 2.03	$ 1.99
Cash distributions paid or payable												$ 340,084	$ 314,135	$ 299,967
Distributions paid as percentage of ordinary income												94.69%	93.23%	90.67%
Distributions paid as a percentage of return of capital												4.39%	6.17%	8.58%
Distributions paid as a percentage of qualified dividend												0.92%	0.60%	0.75%
Common shares | Subsequent event
Common Share Distributions
Cash distributions paid or payable (in dollars per share)		$ 0.52
Dividends, cash amount declared		$ 85,460
Distributions per common share (in dollars per share)	$ 0.52
RMR Inc
Common Share Distributions
Cash distributions paid or payable (in dollars per share)			$ 0.0166
Number of shares distributed to the shareholders (in shares)			2,515,344
Non cash distributions per common share (in dollars per share)			$ 0.1974