Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				1 Months Ended	3 Months Ended				1 Months Ended		1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012 SIR	Jan. 31, 2012 Acquisition	Mar. 31, 2012 Acquisition	Mar. 31, 2012 Unsecured revolving credit facility	Mar. 31, 2011 Unsecured revolving credit facility	Mar. 31, 2012 Unsecured revolving credit facility SIR	Jan. 31, 2012 Senior Notes, due 2012 at 6.95%	Mar. 31, 2012 Mortgage notes maturing from 2012 through 2027 property	Feb. 29, 2012 Mortgage Notes Payable, due 2012 at 7.31%
Indebtedness
Debt repaid at par										$ 150,680
Interest rate stated percentage					6.29%					6.95%		7.31%
Gain (loss) on early extinguishment of debt	(67)									(67)
Assumption of mortgage debt	147,872				147,872	147,872
Fair value of mortgages assumed					160,330
Repayments on maturity	369,082	194,318										5,404
Maximum borrowing capacity				500,000			750,000		500,000
Variable rate basis							LIBOR		LIBOR
Basis points on variable rate (as a percent)							1.25%
Revolving credit facility, average interest rate (as a percent)							1.50%	2.30%	1.50%
Amount outstanding									227,000
Available borrowing capacity							750,000		273,000
Cost of properties mortgaged	7,485,504		7,244,232								1,055,325
Mortgaged properties aggregate net book value	6,515,601		6,310,062								934,365
Mortgage notes payable, net	$ 784,454		$ 632,301								$ 784,454
Number of properties mortgaged											22