Credit Facility (Details) (USD $)			9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Unsecured revolving credit facility	Sep. 30, 2012 Unsecured revolving credit facility	Oct. 31, 2013 Unsecured revolving credit facility Subsequent event	Oct. 29, 2013 Unsecured revolving credit facility Subsequent event
Credit facility
Maximum borrowing capacity			$ 1,000,000,000		$ 1,500,000,000	$ 1,500,000,000
Term of extension option			1 year
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%
Increase in the maximum borrowing capacity after amendment					500,000,000	500,000,000
Unamortized debt issuance costs			6,400,000
Current borrowing capacity available			531,600,000			1,400,000,000
Outstanding balance	$ 468,400,000	$ 158,000,000
Weighted average borrowing rate during the period (as a percent)			1.30%	1.60%
Effective interest rate (as a percent)			1.30%