Credit Facility (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	May 31, 2012	Apr. 30, 2012
Credit facility
Outstanding balance	$ 158,000,000	$ 237,400,000
Unsecured revolving credit facility
Credit facility
Maximum borrowing capacity				1,000,000,000	425,000,000
Term of extension option	1 year
Line of credit facility, variable reference rate	LIBOR
Variable interest rate, basis points spread over variable reference rate (as a percent)	1.075%
Line of credit facility, commitment fee basis points (as a percent)	0.175%
Line of credit facility, all-in drawn variable interest rate (as a percent)	1.25%
Unamortized debt issuance costs	5,900,000			7,100,000
Remaining origination costs from replaced credit facility	2,200,000
Current borrowing capacity available	842,000,000
Outstanding balance	$ 158,000,000	$ 237,400,000
Average borrowing rate during the period (as a percent)	1.60%	2.10%	1.30%
Effective interest rate (as a percent)	1.30%