Employee Benefits - Amounts Recognized in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Total, beginning balance	$ (2,413)	$ (2,231)
Total, recognition of net periodic benefits expense	162	175
Total, deferrals	(218)	(357)
Total, net change in AOCL	(56)	(182)
Total, ending balance	(2,469)	(2,413)
Combined Pension Plan
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Net actuarial (loss) gain, beginning balance	(3,046)	(2,973)
Net actuarial (loss) gain, recognition of net periodic benefits expense	203	224
Net actuarial (loss) gain, deferrals	(150)	(297)
Net actuarial (loss) gain, net change in AOCL	53	(73)
Net actuarial (loss) gain, ending balance	(2,993)	(3,046)
Prior service benefit (cost), beginning balance	47	46
Prior service benefit (cost), recognition of net periodic benefits expense	(9)	(8)
Prior service benefit (cost), deferrals	3	9
Prior service benefit (cost), net change in AOCL	(6)	1
Prior service benefit (cost), ending balance	41	47
Deferred income tax benefit (expense), beginning balance	770	754
Deferred income tax benefit (expense), recognition of net periodic benefits expense	(47)	(53)
Deferred income tax benefit (expense), deferrals	32	69
Deferred income tax benefit (expense), net change in AOCL	(15)	16
Deferred income tax benefit (expense), ending balance	755	770
Total, beginning balance	(2,229)	(2,173)
Total, recognition of net periodic benefits expense	147	163
Total, deferrals	(115)	(219)
Total, net change in AOCL	32	(56)
Total, ending balance	(2,197)	(2,229)
Post-Retirement Benefit Plans
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Net actuarial (loss) gain, beginning balance	(175)	7
Net actuarial (loss) gain, recognition of net periodic benefits expense	0	0
Net actuarial (loss) gain, deferrals	(171)	(182)
Net actuarial (loss) gain, net change in AOCL	(171)	(182)
Net actuarial (loss) gain, ending balance	(346)	(175)
Prior service benefit (cost), beginning balance	(71)	(87)
Prior service benefit (cost), recognition of net periodic benefits expense	16	16
Prior service benefit (cost), deferrals	35	0
Prior service benefit (cost), net change in AOCL	51	16
Prior service benefit (cost), ending balance	(20)	(71)
Curtailment loss, beginning balance	0
Curtailment loss, recognition of net periodic benefits expense	4
Curtailment loss, deferrals	0
Curtailment loss, net change in AOCL	4
Curtailment loss, ending balance	4	0
Deferred income tax benefit (expense), beginning balance	62	22
Deferred income tax benefit (expense), recognition of net periodic benefits expense	(5)	(4)
Deferred income tax benefit (expense), deferrals	33	44
Deferred income tax benefit (expense), net change in AOCL	28	40
Deferred income tax benefit (expense), ending balance	90	62
Total, beginning balance	(184)	(58)
Total, recognition of net periodic benefits expense	15	12
Total, deferrals	(103)	(138)
Total, net change in AOCL	(88)	(126)
Total, ending balance	$ (272)	$ (184)