Credit Facility (Details)	1 Months Ended	9 Months Ended
	Oct. 31, 2018 USD ($) extension	Sep. 30, 2018 USD ($) extension	Sep. 30, 2017	Dec. 31, 2017 USD ($)
Credit facility
Credit facility origination costs		$ 2,206,000		$ 4,366,000
Outstanding balance		$ 774,000,000		110,000,000
Revolving Credit Facility
Credit facility
Commitment fee (as a percent)		0.125%
Current borrowing capacity		$ 1,200,000,000
Outstanding balance		$ 774,000,000.0		$ 110,000,000.0
Weighted average borrowing rate during the period (as a percent)		2.80%	1.90%
Weighted average interest rate (as a percent)		3.10%		4.50%
Revolving Credit Facility | Other assets, net
Credit facility
Credit facility origination costs		$ 2,200,000
Revolving Credit Facility | LIBOR
Credit facility
Basis spread on variable rate (as a percent)		0.85%
All-in drawn variable interest rate (as a percent)		0.975%
Variable reference rate		LIBOR
Revolving Credit Facility | Unsecured debt
Credit facility
Number of extensions | extension		2
Term of extension option		6 months
Accordion expansion option		$ 1,000,000,000.0
Credit Facility | Unsecured debt
Credit facility
Credit facility borrowing capacity		2,250,000,000
Credit Facility | Revolving Credit Facility | Unsecured debt
Credit facility
Credit facility borrowing capacity		$ 2,000,000,000.0
Amended Credit Facility | Unsecured debt | Subsequent event
Credit facility
Credit facility borrowing capacity	$ 3,250,000,000
Amended Credit Facility | Revolving Credit Facility | Subsequent event
Credit facility
Commitment fee (as a percent)	0.125%
Amended Credit Facility | Revolving Credit Facility | LIBOR | Subsequent event
Credit facility
Basis spread on variable rate (as a percent)	0.775%
All-in drawn variable interest rate (as a percent)	0.90%
Amended Credit Facility | Revolving Credit Facility | Unsecured debt | Subsequent event
Credit facility
Credit facility borrowing capacity	$ 3,000,000,000.0
Number of extensions | extension	2
Term of extension option	6 months
Accordion expansion option	$ 1,000,000,000.0