Credit Facility (Details) $ in Thousands	12 Months Ended	24 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2015 USD ($) item	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Credit facility
Outstanding balance	$ 238,000	$ 238,000		$ 223,000
Credit Facility
Credit facility
Maximum borrowing capacity					$ 1,500,000
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	$ 10,200	10,200
Current borrowing capacity available	$ 1,760,000	$ 1,760,000
Average borrowing rate during the period (as a percent)		1.20%
Effective interest rate (as a percent)	1.20%	1.20%
Credit Facility | Unsecured debt
Credit facility
Maximum borrowing capacity			$ 2,000,000
Line Of Credit Facility Number Of Extensions | item	2	2
Term of extension option	6 months
Increase in the maximum borrowing capacity after amendment	$ 1,000,000	$ 1,000,000
Credit facility origination costs incurred	$ 9,100