Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended	0 Months Ended	3 Months Ended	0 Months Ended	1 Months Ended		3 Months Ended	0 Months Ended	1 Months Ended
	Feb. 15, 2013 Series D	Mar. 31, 2013 Series D	Feb. 15, 2013 Series E	Mar. 31, 2013 Series E	Feb. 21, 2013 Common Shares	Apr. 30, 2013 Common Shares	Mar. 31, 2013 Common Shares	Mar. 31, 2013 Common Shares	Feb. 19, 2013 Common Shares SIR	Apr. 30, 2013 Common Shares SIR
Shareholders' Equity
Common shares issued							34,500,000
Common stock sold pursuant to the underwriters' option to purchase additional shares (in shares)							4,500,000
Price per common share (in dollars per share)							$ 19.00	$ 19.00
Gross proceeds received on issuance of common shares							$ 655,500
Net proceeds received on issuance of common shares							627,074
Cash distributions paid on preferred shares (in dollars per share)	$ 0.4063		$ 0.4531
Distribution paid or declared on preferred shares	6,167		4,984
Additional distributions on common stock declared (in dollars per share)					$ 0.25	$ 0.25			$ 0.42	$ 0.44
Distributions paid or declared on common shares					$ 20,951	$ 29,600			$ 16,499	$ 17,300
Distributions on preferred stock declared (in dollars per share)		$ 0.4063		$ 0.4531				$ 0.25