Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 13, 2013 RMR Award Plans Officers and employees of RMR	Jul. 02, 2013 Series C	May 15, 2013 Series C	Feb. 15, 2013 Series C	Oct. 15, 2013 Series D	Jul. 15, 2013 Series D	Apr. 15, 2013 Series D	Jan. 15, 2013 Series D	Aug. 23, 2013 Common Shares	May 24, 2013 Common Shares	May 15, 2013 Common Shares item	Mar. 22, 2013 Common Shares	Feb. 22, 2013 Common Shares	Oct. 10, 2013 Subsequent event Common Shares
Distributions
Total distribution paid to preferred shareholders							$ 2,931	$ 2,931	$ 5,166	$ 5,166	$ 5,166	$ 5,166	$ 65,681	$ 65,677			$ 58,110
Distribution to preferred shareholders (in dollars per share)							$ 0.4375	$ 0.4375	$ 0.4453125	$ 0.4453125	$ 0.4453125	$ 0.4453125
Distribution declared per share (in dollars per share)																		$ 0.48
Distribution to common shareholders (in dollars per share)													$ 0.47	$ 0.47			$ 0.47
Common Share Issuances
Common stock, shares issued (in shares)																16,100,000
Common stock, issue price (in dollars per share)																$ 25.55
Net proceeds from sale of common shares			393,475													393,475
Common shares granted to trustees in period															2,000
Common stock price per share which is the closing price at New York Stock Exchange (in dollars per share)															$ 30.69
Number of Trustees															5
Common shares granted under equity compensation plan					84,125
Grant date value (in dollars per share)					$ 27.42
Number of shares redeemed						6,700,000
Dividend rate (as a percent)						7.00%
Liquidation preference (in dollars per share)						$ 25
Reduced net income available for common shareholders on the excess of liquidation preference of the redeemed shares over carrying amount	$ 5,627	$ 5,040	$ 5,627	$ 7,984		$ 5,627