Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended												1 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 SIR	Dec. 31, 2012 SIR	Dec. 31, 2013 2014	Dec. 31, 2013 2015	Dec. 31, 2013 2016	Dec. 31, 2013 2017	Dec. 31, 2013 Award Plan	Dec. 31, 2012 Award Plan	Dec. 31, 2011 Award Plan	Jan. 31, 2014 Distribution declared
Distributions
Distributions on common stock declared (in dollars per share)	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.50	$ 0.50	$ 0.50	$ 1.00	$ 1.75	$ 2.00										$ 0.25
Distributions paid or declared on common shares																					$ 29,597
Ordinary income (as a percent)									0.00%	61.72%	65.90%
Number of Shares
Unvested shares at the beginning of the period																		114,703	103,615	70,520
Granted in the period (in shares)												2,000						83,450	83,617	83,050
Vested in the period (in shares)														(47,868)	(40,593)	(27,843)	(14,610)	(66,639)	(71,244)	(49,955)
Forfeited in the period (in shares)																		(600)	(1,285)
Unvested shares at the end of the period																		130,914	114,703	103,615
Weighted Average Grant Date Fair Value
Unvested shares at the beginning of the period (in dollars per share)																		$ 19.39	$ 23.32	$ 27.96
Granted in the period (in dollars per share)																		$ 23.29	$ 15.99	$ 20.76
Vested in the period (in dollars per share)																		$ 23.06	$ 15.75	$ 20.79
Forfeited in the period (in dollars per share)																		$ 22.49	$ 18.74
Unvested shares at the end of the period (in dollars per share)																		$ 20.70	$ 19.39	$ 23.32
Estimated future compensation expense for the unvested shares																		3,052
Common stock shares closing price per share																		$ 23.31
Weighted average period over which the compensation expense will be recorded																		22 months
Compensation expense												$ 355	$ 507					$ 1,758	$ 1,179	$ 1,139
Shares available for issuance under the Award Plan																		2,833,533
Return of capital (as a percent)									100.00%	38.28%	23.54%
Internal revenue code section 1250 gain (as a percent)									0.00%	0.00%	7.34%
Capital gain (as a percent)									0.00%	0.00%	3.22%