Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended				1 Months Ended	9 Months Ended				0 Months Ended
	Oct. 03, 2013	Aug. 21, 2013	May 21, 2013	Feb. 19, 2013	Jan. 31, 2013	Sep. 30, 2013	Sep. 30, 2012	May 09, 2013 item	Dec. 31, 2012	Sep. 13, 2013 RMR	Mar. 27, 2013 RMR
Distributions
Distribution to common shareholders (in dollars per share)	$ 0.39	$ 0.39	$ 0.39	$ 0.39
Distribution to common shareholders	$ 73,386	$ 73,353	$ 73,349	$ 68,857		$ 215,559	$ 190,680
Common shares issued in payment of an incentive fee for services rendered from RMR											21,968
Value of common shares issued in payment of an incentive fee for services rendered from RMR											582
Common shares issued in public offering					11,500,000
Net proceeds from issuance of common shares issued in public offering					$ 262,068	$ 261,813	$ 287,052
Common shares of beneficial interest
Common shares of beneficial interest granted								2,000
Common shares of beneficial interest, par value (in dollars per share)						$ 0.01		$ 0.01	$ 0.01
Common stock price per share which is the closing price at New York Stock Exchange (in dollars per share)								$ 28.64
Number of trustees								5
Common shares granted to the entity's officers and certain employees										82,600
Closing price of common shares granted to the entity's officers and certain employees (in dollars per share)										$ 22.86