Credit Facility (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Credit facility
Credit facility origination costs, net	$ 4,366	$ 7,303
Outstanding balance	$ 110,000	1,120,000
Credit Facility
Credit facility
Commitment fee (as a percent)	0.125%
Current borrowing capacity	$ 1,890,000
Outstanding balance	$ 110,000	$ 1,120,000
Weighted average borrowing rate during the period (as a percent)	2.00%	1.40%
Weighted average interest rate at the end of the period (as a percent)	4.50%	1.70%
Credit Facility | Other assets, net
Credit facility
Credit facility origination costs, net	$ 4,400
Credit Facility | Unsecured debt
Credit facility
Maximum borrowing capacity	$ 2,000,000
Number of extensions | item	2
Term of extension option	6 months
Accordion expansion option	$ 1,000,000
Credit Facility | LIBOR
Credit facility
Variable reference rate	LIBOR
Basis spread on variable rate (as a percent)	0.85%
All-in drawn variable interest rate (as a percent)	0.975%