Credit Facility (Details) (USD $)			3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Unsecured revolving credit facility	Mar. 31, 2013 Unsecured revolving credit facility	Apr. 03, 2014 Unsecured revolving credit facility Subsequent event
Credit facility
Maximum borrowing capacity			$ 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%
Unamortized debt issuance costs			6,400,000
Current borrowing capacity available			759,900,000		1,290,000,000
Outstanding balance	$ 740,100,000	$ 128,000,000
Average borrowing rate during the period (as a percent)			1.20%	1.30%
Effective interest rate (as a percent)			1.20%