Credit Facility (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) item	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Credit facility
Outstanding balance	$ 430,000		$ 223,000
Credit Facility
Credit facility
Maximum borrowing capacity		$ 1,500,000	$ 1,500,000
Average borrowing rate during the period (as a percent)		1.20%
Credit Facility | Unsecured debt
Credit facility
Maximum borrowing capacity	$ 2,000,000
Number of extensions available | item	2
Term of extension option	6 months
Variable reference rate	LIBOR
Variable interest rate, basis points spread over variable reference rate (as a percent)	0.90%
Line of credit facility, commitment fee basis points (as a percent)	0.15%
Line of credit facility, all-in drawn variable interest rate (as a percent)	1.05%
Unamortized debt issuance costs	$ 12,800
Credit facility origination costs incurred	10,200
Remaining origination costs from replaced credit facility and term loan facility	2,600
Current borrowing capacity available	$ 1,570,000
Average borrowing rate during the period (as a percent)	1.30%
Effective interest rate (as a percent)	1.20%
Increase in the maximum borrowing capacity after amendment	$ 1,000,000