Partnership Capital Subject to Mandatory Redemption - Roll Forward of Outstanding Partnership Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Partners Capital Notes [Abstract]
Partnership loans outstanding at beginning of year	$ 360	$ 332	$ 297
Partnership loans issued during the year	163	164	170
Repayment of Partnership loans during the year	(182)	(136)	(135)
Partnership loans outstanding at end of year	$ 341	$ 360	$ 332