Cash Flow Information - Schedule of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Cash paid for interest	$ 103	$ 157	$ 125
Cash paid for taxes	11	10	10
Non-cash activities:
Issuance of general partnership interests through partnership loans in current year	163	164	170
Repayment of partnership loans through distributions from partnership capital in current year	182	136	135
Declared distributions for retired partnership capital in current year but unpaid at year end	$ 145	$ 113	$ 97