Credit Facility (Details) (USD $)			6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Unsecured revolving credit facility	Jun. 30, 2013 Unsecured revolving credit facility
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			5,700,000
Current borrowing capacity available			1,430,000,000
Outstanding balance	$ 70,800,000	$ 128,000,000
Average borrowing rate during the period (as a percent)			1.20%	1.30%
Effective interest rate (as a percent)			1.20%