Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended							3 Months Ended		1 Months Ended	3 Months Ended				3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 5.75% Senior Notes due February 15, 2014	Mar. 31, 2013 6.40% Senior Notes due February 15, 2015	Mar. 31, 2013 5.75% Senior Notes due November 1, 2015	Mar. 31, 2013 6.25% Senior Notes due August 15, 2016	Mar. 31, 2013 Unsecured revolving credit facility	Mar. 31, 2012 Unsecured revolving credit facility	Mar. 31, 2012 Unsecured revolving credit facility SIR	Mar. 31, 2013 Unsecured revolving credit facility SIR	Feb. 28, 2013 Unsecured revolving credit facility SIR	Dec. 31, 2012 Unsecured revolving credit facility SIR	Mar. 31, 2013 Mortgage notes maturing from 2014 through 2026 property	Mar. 31, 2013 Unsecured term loan	Mar. 31, 2012 Unsecured term loan	Mar. 31, 2013 Unsecured term loan SIR	Jul. 31, 2012 Unsecured term loan SIR
Indebtedness
Principal	$ 670,295			$ 145,612	$ 152,560	$ 111,227	$ 260,896
Purchase Price	726,151			148,746	164,140	121,047	292,218
Loss on early extinguishment of debt	(60,027)	(67)
Maximum borrowing capacity								750,000				750,000	500,000
Maximum amount of borrowing								1,500,000										700,000
Maturity date extension period available								1 year			1 year
Interest rate stated percentage				5.75%	6.40%	5.75%	6.25%								2.10%		1.80%
Increased maximum borrowing capacity available under specified circumstances															1,000,000
Variable rate basis								LIBOR			LIBOR				LIBOR		LIBOR
Basis points on variable rate (as a percent)								1.50%			1.30%				1.85%		1.55%
Facility fee of lending commitments under our revolving credit facility (as a percent)								0.35%			0.30%
Interest rate payable (as a percent)								1.70%			1.50%
Average interest rate (as a percent)								1.70%	1.50%	1.50%	1.50%				2.10%	1.80%	1.80%
Amount outstanding								135,000			238,000
Available Borrowing Capacity								615,000			512,000
Loan maturing in December 2016															500,000
Principal balance																		350,000
Number of properties mortgaged														25
Cost of properties mortgaged	7,989,769		7,829,409											1,329,835
Mortgaged properties aggregate net book value	6,942,072		6,821,803											1,188,911
Mortgage notes payable, net	$ 980,985		$ 984,827											$ 980,985