Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended			0 Months Ended						3 Months Ended		0 Months Ended			0 Months Ended
	Mar. 25, 2014	Mar. 31, 2014	Dec. 31, 2013	Mar. 07, 2014 RMR	Feb. 07, 2014 RMR	May 07, 2014 RMR Subsequent Event	Apr. 07, 2014 RMR Subsequent Event	Mar. 25, 2014 Equity compensation plan Officers and employees RMR	Feb. 15, 2014 Series D	Mar. 31, 2014 Series D	Dec. 31, 2013 Series D	Feb. 15, 2014 Series E	Mar. 31, 2014 Series E	Dec. 31, 2013 Series E	Feb. 21, 2014 Common Shares	Jan. 28, 2014 Common Shares Trustees trustee	Mar. 25, 2014 Common Shares Equity compensation plan Officers and employees RMR
Shareholders' Equity
Common shares issued				10,625	12,187	11,275	11,410
Shares of beneficial interest granted																2,000
Par value of shares of beneficial interest (in dollars per share)		$ 0.01	$ 0.01													$ 0.01
Price per common share (in dollars per share)																$ 23.46
Number of former trustees to whom common shares granted																2
Shares issued																	130,914
Compensation expense								$ 3,412
Cash distributions paid on preferred shares (in dollars per share)									$ 0.4063			$ 0.4531
Distribution paid or declared on preferred shares	0								6,167			4,984
Additional distributions on common stock declared (in dollars per share)															$ 0.25
Distributions paid or declared on common shares															$ 29,597
Preferred shares outstanding										15,180,000	15,180,000		11,000,000	11,000,000
Liquidation preference (in dollars per share)										$ 25.00
Percentage of average closing market price of the entity's common stock based on which ratio for conversion of preferred shares into common shares is determined										98.00%
Number of consecutive trading days considered for calculating the average of the closing sale prices of the common shares										5 days