Credit Facility (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Credit facility
Outstanding balance	$ 370,000,000		$ 223,000,000
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	3,400,000
Current borrowing capacity available	$ 1,130,000,000
Average borrowing rate during the period (as a percent)	1.20%	1.20%
Effective interest rate (as a percent)	1.20%