Related Party Debt - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Line Of Credit Facility [Line Items]
Interest and fee expenses	$ 22,000,000	$ 26,000,000	$ 69,000,000	$ 67,000,000
Interest paid	22,000,000	$ 23,000,000	71,000,000	$ 62,000,000
Long-term debt, carrying value	2,694,000,000		2,694,000,000		$ 2,694,000,000
Long-term debt, fair value	2,919,000,000		2,919,000,000		2,825,000,000
Borrowing capacity	3,590,000,000		3,590,000,000		3,590,000,000
Ten Year Fixed Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Borrowing capacity	600,000,000		600,000,000		600,000,000
Seven Year Fixed Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Borrowing capacity	600,000,000		600,000,000		600,000,000
Five Year Revolver due July 2023 | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Borrowing capacity	760,000,000		760,000,000		760,000,000
Five Year Fixed Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Borrowing capacity	$ 600,000,000		$ 600,000,000		$ 600,000,000