Credit Facility (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) item	Sep. 30, 2015	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)
Credit facility
Outstanding balance	$ 1,082,000		$ 238,000
Credit Facility
Credit facility
Maximum borrowing capacity					$ 1,500,000
Credit Facility | Unsecured debt
Credit facility
Maximum borrowing capacity				$ 2,000,000
Number of extensions available | item	2
Term of extension option	6 months
Increase in the maximum borrowing capacity after amendment	$ 1,000,000
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	$ 8,000
Credit facility origination costs incurred	9,100
Current borrowing capacity available	918,000
Outstanding balance	$ 1,080,000		$ 238,000
Average borrowing rate during the period (as a percent)	1.40%	1.20%
Weighted average interest rate at the end of the period (as a percent)	1.40%