Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended								12 Months Ended													1 Months Ended
	Dec. 31, 2009	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 SIR	Dec. 31, 2012 SIR Officers and employees RMR	Dec. 31, 2012 SIR Trustees item	Dec. 31, 2012 2013	Dec. 31, 2012 2014	Dec. 31, 2012 2015	Dec. 31, 2012 2015 thereafter	Dec. 31, 2012 Award Plan	Dec. 31, 2011 Award Plan	Dec. 31, 2010 Award Plan	Jan. 31, 2013 Distribution declared	Jan. 31, 2013 Distribution declared SIR
Distributions
Number of shares granted under the Award Plan														22,592	2,000					82,332	83,050	48,625
Number of trustees															5
Aggregate market value of shares granted under the Award Plan													$ 507
Distributions on common stock declared (in dollars per share)	$ 0.48	$ 0.25	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 1.75	$ 2.00	$ 1.48	$ 0.91										$ 0.25	$ 0.42
Distributions paid or declared on common shares																							20,951	16,499
Ordinary income (as a percent)										61.72%	65.90%	82.82%	100.00%
Number of Shares
Unvested shares at the beginning of the period																				103,615	70,520	53,565
Granted in the period (in shares)														22,592	2,000					82,332	83,050	48,625
Vested in the period (in shares)																(40,569)	(34,018)	(26,543)	(13,573)	(71,244)	(49,955)	(31,670)
Unvested shares at the end of the period																				114,703	103,615	70,520
Weighted Average Grant Date Fair Value
Unvested shares at the beginning of the period (in dollars per share)																				$ 23.32	$ 27.96	$ 28.76
Granted in the period (in dollars per share)																				$ 15.99	$ 20.76	$ 27.96
Vested in the period (in dollars per share)																				$ 15.75	$ 20.79	$ 28.29
Unvested shares at the end of the period (in dollars per share)																				$ 19.39	$ 23.32	$ 27.96
Estimated future compensation expense for the unvested shares																				1,817
Common stock shares closing price per share																				$ 15.84
Weighted average period over which the compensation expense will be recorded																				22 months
Compensation expense																				$ 1,179	$ 1,139	$ 1,034
Shares available for issuance under the Award Plan																				2,916,383
Return of capital (as a percent)										38.28%	23.54%	17.18%
Internal revenue code section 1250 gain (as a percent)										0.00%	7.34%	0.00%
Capital gain (as a percent)										0.00%	3.22%	0.00%