Credit Facility (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Line of Credit Facility [Line Items]
Revolving unsecured credit facility	$ 425,000,000
Expiration date	March 2014
Available one-year extension options	two, one-year extension options
Variable interest rate, LIBOR plus basis points	LIBOR, plus 185 basis points
Line of credit facility, commitment fee basis points	35 basis points
Line of credit facility, all-in drawn variable interest rate	220 basis points over LIBOR
Unamortized debt issuance expense	4,200,000
Balance of credit facility origination costs	2,800,000		3,100,000
Line of Credit Borrowing Capacity	382,000,000
Line of Credit Outstanding Balance	$ 43,000,000		$ 237,400,000
Average borrowing rate during period (in hundredths)	2.10%	2.10%