Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
2012 Equity Compensation Plan
Shareholders' Equity
Shares of common shares reserved to be issued under share award plans	3,000,000
Remaining shares of common shares reserved to be issued under share award plans	2,915,983
Award Plans
Shareholders' Equity
Shares of common shares reserved to be issued under share award plans	3,128,791
Common shares granted under equity compensation plan	84,017	77,300	63,900
Number of Shares
Unvested shares, beginning of year	114,110	93,432	74,062
Shares granted	84,017	77,300	63,900
Shares vested	(70,473)	(56,622)	(44,530)
Unvested shares, end of year	127,654	114,110	93,432
Weighted Average Grant Date Fair Value
Unvested shares, beginning of year (in dollars per share)	$ 22.70	$ 22.31	$ 23.20
Shares granted (in dollars per share)	$ 25.09	$ 24.20	$ 22.89
Shares vested (in dollars per share)	$ 24.38	$ 23.76	$ 22.63
Unvested shares, end of year (in dollars per share)	$ 24.88	$ 22.70	$ 22.31
Unvested shares, scheduled to vest
2013	47,763
2014	37,863
2015	27,524
2016	14,504
Estimated future compensation expense	$ 2,987
Closing price of common shares (in dollars per share)	$ 23.42
Weighted average period over which the compensation expense will be recorded	22 months
Compensation expense	1,888	1,405	1,340
Award Plans | Trustees
Shareholders' Equity
Common shares granted under equity compensation plan	2,000	2,000	2,000
Market value of common shares awarded	256	243	260
Market value of common stock issued per trustee	51	49	52
Number of Shares
Shares granted	2,000	2,000	2,000
Award Plans | RMR | Officers and employees of RMR
Shareholders' Equity
Common shares granted under equity compensation plan	74,017	67,300	53,900
Market value of common shares awarded	$ 1,852	$ 1,628	$ 1,186
Vesting period for shares awarded	5 years
Number of Shares
Shares granted	74,017	67,300	53,900