Partnership Capital Subject to Mandatory Redemption - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 29, 2021	Jan. 31, 2020	Jan. 25, 2019	Jan. 12, 2018	Dec. 31, 2017
Partners Capital Account [Line Items]
Period of loans made by Partnership to general partners	1 year
Outstanding amount of partner loans financed through the Partnership	$ 341	$ 360	$ 332					$ 297
Interest income from outstanding amount of general partner loan	$ 14	$ 21	$ 17
Limited partnership's minimum annual return rate	7.50%	7.50%	7.50%
Limited partnership's minimum return, value	$ 93	$ 94	$ 67
2018 Limited Partnership Offering [Member]
Partners Capital Account [Line Items]
Limited partnership amount registered							$ 450
Limited partnership interests issued					$ 1	$ 380
Remaining limited partnership interests that may be issued	$ 66
2018 Limited Partnership Offering [Member] | Subsequent Event [Member]
Partners Capital Account [Line Items]
Limited partnership interests issued				$ 3