Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified			3 Months Ended	12 Months Ended	1 Months Ended														1 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 SIR property item	Dec. 31, 2012 SIR	Jul. 31, 2012 Series C	May 31, 2012 Series C	Feb. 29, 2012 Series C	Jul. 31, 2012 Series D	May 31, 2012 Series D	Feb. 29, 2012 Series D	Jul. 31, 2012 Series E	May 31, 2012 Series E	Feb. 29, 2012 Series E	Jul. 31, 2012 Common Stock	May 31, 2012 Common Stock trustee	Feb. 29, 2012 Common Stock	Jul. 18, 2012 Common Stock	May 08, 2012 Common Stock	Jul. 31, 2012 Common Stock SIR
Shareholders' Equity
Cash distributions paid on preferred shares (in dollars per share)						$ 0.4453	$ 0.4453		$ 0.4063	$ 0.4063		$ 0.4531	$ 0.4531
Distribution paid or declared on preferred shares					$ 2,672	$ 2,672	$ 2,672	$ 6,167	$ 6,167	$ 6,167	$ 4,984	$ 4,984	$ 4,984
Cash distributions paid on common shares (in dollars per share)															$ 0.50	$ 0.50
Distributions paid or declared on common shares														$ 41,900	$ 41,861	$ 41,861			$ 15,300
Distributions on common stock declared (in dollars per share)			$ 0.40	$ 1.60										$ 0.50					$ 0.49
Additional distributions on common stock declared (in dollars per share)			0.09
Number of days as a public company in the prior quarter for which additional dividend was declared			20
Distributions on preferred stock declared (in dollars per share)					$ 0.4453			$ 0.4063			$ 0.4531
Share Issuances
Common shares of beneficial interest issued to trustees as part of annual compensation (in shares)														2,000	2,000
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01															$ 0.01	$ 0.01
Stock price per share which is the closing price at NYSE (in dollars per share)																	$ 19.27	$ 18.74
Number of trustees receiving shares as part of annual compensation															5