Employee Benefits - Reconciliation of Beginning and Ending Balance of Pension Plan Assets Measured at Fair Value (Detail) - Pension - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	$ 19,451	$ 17,816
End of year	20,128	19,451
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	1,850	1,785
Actual gain (loss) on plan assets	204	61
Purchases (sales)	(188)	226
Transfers out	(359)	(222)
End of year	1,507	1,850
Level 3 | Equity securities
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	3	13
Actual gain (loss) on plan assets	5	1
Purchases (sales)	(7)	(11)
Transfers out	1	0
End of year	2	3
Level 3 | Corporate bonds
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	145	277
Actual gain (loss) on plan assets	(8)	(1)
Purchases (sales)	39	18
Transfers out	0	(149)
End of year	176	145
Level 3 | International bonds
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	26	18
Actual gain (loss) on plan assets	3	(1)
Purchases (sales)	(9)	9
Transfers out	0	0
End of year	20	26
Level 3 | Real estate
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	810	727
Actual gain (loss) on plan assets	146	30
Purchases (sales)	(146)	53
Transfers out	(53)	0
End of year	757	810
Level 3 | Private equity
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	737	664
Actual gain (loss) on plan assets	57	32
Purchases (sales)	(134)	41
Transfers out	(246)	0
End of year	414	737
Level 3 | Hedge funds
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning of year	129	86
Actual gain (loss) on plan assets	1	0
Purchases (sales)	69	116
Transfers out	(61)	(73)
End of year	$ 138	$ 129