Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended		0 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended
	Feb. 21, 2014	Mar. 31, 2014	Mar. 31, 2013	Apr. 14, 2014 Subsequent event	Apr. 02, 2014 Subsequent event	Apr. 30, 2014 Subsequent event	Mar. 31, 2014 RMR	Apr. 30, 2014 RMR Subsequent event
Distributions
Distribution to common shareholders (in dollars per share)	$ 0.39				$ 0.39
Distribution to common shareholders	$ 73,386	$ 73,386	$ 68,857	$ 73,393
Common shares issued as a part of compensation under business management agreement							19,937	10,342
Common shares issued in public offering						15,525,000
Net proceeds after underwriting discounts from issuance of common shares issued in public offering			$ 261,859			$ 323,318