Reconciliation of Current Period Changes, Net of Applicable Income Taxes, for Derivatives Qualifying as Hedges (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments [Abstract]
Derivatives qualifying as effective accounting hedges as of January 1	$ 2,002	$ 1,781	$ 2,065
Cumulative effect of changes in accounting:
Stranded tax effects	0	0	12
Changes to the hedge accounting model, net of deferred taxes of $—, $—and $(1)	0	0	2
Accumulated other comprehensive income	4,425	3,433	2,044	$ 3,027
Current period increases (decreases) in fair value, net of deferred taxes of $(95), $(87) and $50	344	331	(194)
Reclassification to net (income), net of deferred taxes of $73, $60 and $58	(135)	(110)	(104)
Derivatives qualifying as effective accounting hedges as of December 31	2,211	2,002	1,781
Cumulative effect of changes in accounting
Cumulative effect of changes in accounting:
Accumulated other comprehensive income				$ 131
Cumulative effect of changes in accounting | Derivatives qualifying as hedges
Cumulative effect of changes in accounting:
Accumulated other comprehensive income	$ 0	$ 0	$ 14