Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified			0 Months Ended				1 Months Ended	0 Months Ended			1 Months Ended	0 Months Ended				1 Months Ended		0 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Jul. 02, 2013 SIR	Aug. 15, 2013 Series D	May 15, 2013 Series D	Feb. 15, 2013 Series D	Oct. 31, 2013 Series D	Aug. 15, 2013 Series E	May 15, 2013 Series E	Feb. 15, 2013 Series E	Oct. 31, 2013 Series E	Aug. 23, 2013 Common Shares	May 22, 2013 Common Shares	May 14, 2013 Common Shares trustee	Feb. 21, 2013 Common Shares	Oct. 31, 2013 Common Shares	Mar. 31, 2013 Common Shares	Sep. 13, 2013 Common Shares Equity compensation plan
Shareholders' Equity
Common shares issued			10,500,000														34,500,000
Common stock sold pursuant to the underwriters' option to purchase additional shares (in shares)																	4,500,000
Shares of beneficial interest granted														2,000
Price per common share (in dollars per share)														$ 20.13			$ 19.00	$ 23.72
Common shares granted under equity compensation plan																		73,450
Gross proceeds received on issuance of common shares																	$ 655,500
Net proceeds received on issuance of common shares																	626,904
Cash distributions paid on preferred shares (in dollars per share)				$ 0.4063	$ 0.4063	$ 0.4063		$ 0.4531	$ 0.4531	$ 0.4531
Distribution paid or declared on preferred shares				6,167	6,167	6,167	6,167	4,984	4,984	4,984	4,984
Par value of shares of beneficial interest (in dollars per share)	$ 0.01	$ 0.01												$ 0.01
Additional distributions on common stock declared (in dollars per share)												$ 0.25	$ 0.25		$ 0.25	$ 0.25
Distributions paid or declared on common shares												$ 29,579	$ 29,576		$ 20,951	$ 29,600
Distributions on preferred stock declared (in dollars per share)							$ 0.4063				$ 0.4531
Number of trustees to whom common shares granted														5