Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended				1 Months Ended	3 Months Ended		0 Months Ended
	Apr. 02, 2013	Feb. 19, 2013	May 09, 2012	Feb. 09, 2012	Jan. 31, 2013	Mar. 31, 2013	Mar. 31, 2012	Mar. 27, 2013 RMR
Distributions
Distribution to common shareholders (in dollars per share)	$ 0.39	$ 0.39	$ 0.38	$ 0.38
Distribution to common shareholders	$ 73,349	$ 68,857	$ 61,813	$ 61,806		$ 68,857	$ 61,806
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	$ 261,859