Credit Facility (Details) (USD $)			6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Unsecured revolving credit facility	Jun. 30, 2012 Unsecured revolving credit facility
Credit facility
Maximum borrowing capacity			$ 1,000,000,000
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			7,000,000
Current borrowing capacity available			299,000,000
Outstanding balance	$ 701,000,000	$ 158,000,000
Weighted average borrowing rate during the period (as a percent)			1.30%	1.90%
Effective interest rate (as a percent)			1.30%