Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	12 Months Ended			0 Months Ended		12 Months Ended
	Mar. 27, 2013 RMR	Dec. 31, 2013 2009 Plan	Dec. 31, 2012 2009 Plan	Dec. 31, 2011 2009 Plan	Feb. 07, 2014 2009 Plan RMR	Mar. 27, 2013 2009 Plan RMR	Dec. 31, 2013 2009 Plan Officers and employees RMR	Dec. 31, 2012 2009 Plan Officers and employees RMR	Dec. 31, 2011 2009 Plan Officers and employees RMR	Dec. 31, 2013 2009 Plan Trustees	Dec. 31, 2012 2009 Plan Trustees	Dec. 31, 2011 2009 Plan Trustees
Share Awards
Shares granted for equity compensation plan		58,350	93,058	50,850			48,350	43,917	40,850	2,000	2,000	2,000
Aggregate market value of shares granted under the Award Plan							$ 1,142	$ 1,043	$ 922	$ 266	$ 224	$ 256
Market value of common shares awarded to each trustee (in dollars)										53	45	51
Award vesting period							5 years
Incentive fee payment for services rendered in 2012	485
Shares issued as part of compensation agreement					3,344	20,230
Number of Shares
Unvested shares at the beginning of the period		76,104	64,270	44,320
Granted (in shares)		58,350	93,058	50,850			48,350	43,917	40,850	2,000	2,000	2,000
Shared forfeited (in shares)		(450)	(820)
Vested (in shares)		(48,553)	(80,404)	(30,900)
Unvested shares at the end of the period		85,451	76,104	64,270
Weighted Average Grant Date Fair Value
Unvested shares at the beginning of the period (in dollars per share)		$ 23.71	$ 22.37	$ 26.55
Granted (in dollars per share)		$ 24.12	$ 23.72	$ 22.93
Shared forfeited (in dollars per share)		$ 24.24	$ 23.66
Vested (in dollars per share)		$ 24.37	$ 23.66	$ 23.47
Unvested shares at the end of the period (in dollars per share)		$ 23.66	$ 23.71	$ 22.37
Vesting schedule of unvested shares
2014 (in shares)		32,303
2015 (in shares)		25,573
2016 (in shares)		17,963
2017 (in shares)		9,612
Share Awards, additional disclosures
Estimated future compensation expense for the unvested shares		2,123
Closing share price of the entity's common shares (in dollars per share)		$ 24.85
Weighted average period of recognition of compensation expenses		22 months
Compensation expense		$ 1,269	$ 1,598	$ 1,646
Shares available for issuance under the Award Plan		1,723,212