Employee Benefits - Amounts Recognized in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Total, beginning balance	$ (2,231)	$ (1,966)
Total, recognition of net periodic benefits expense	175	(264)
Total, deferrals	(357)	(1)
Total, net change in AOCL	(182)	(265)
Total, ending balance	(2,413)	(2,231)
Pension Plan
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Net actuarial (loss) gain, beginning balance	(2,973)	(2,892)
Net actuarial (loss) gain, recognition of net periodic benefits expense	224	179
Net actuarial (loss) gain, deferrals	(297)	(260)
Net actuarial (loss) gain, net change in AOCL	(73)	(81)
Net actuarial (loss) gain, ending balance	(3,046)	(2,973)
Prior service benefit (cost), beginning balance	46	54
Prior service benefit (cost), recognition of net periodic benefits expense	(8)	(8)
Prior service benefit (cost), deferrals	9	0
Prior service benefit (cost), net change in AOCL	1	(8)
Prior service benefit (cost), ending balance	47	46
Prior service benefit (cost), beginning balance	754	1,107
Deferred income tax benefit (expense), recognition of net periodic benefits expense	(53)	(418)
Deferred income tax benefit (expense), deferrals	69	65
Deferred income tax benefit (expense), net change in AOCL	16	(353)
Prior service benefit (cost), ending balance	770	754
Total, beginning balance	(2,173)	(1,731)
Total, recognition of net periodic benefits expense	163	(247)
Total, deferrals	(219)	(195)
Total, net change in AOCL	(56)	(442)
Total, ending balance	(2,229)	(2,173)
Other Postretirement Benefits Plan
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Net actuarial (loss) gain, beginning balance	7	(250)
Net actuarial (loss) gain, recognition of net periodic benefits expense	0	0
Net actuarial (loss) gain, deferrals	(182)	257
Net actuarial (loss) gain, net change in AOCL	(182)	257
Net actuarial (loss) gain, ending balance	(175)	7
Prior service benefit (cost), beginning balance	(87)	(107)
Prior service benefit (cost), recognition of net periodic benefits expense	16	20
Prior service benefit (cost), deferrals	0	0
Prior service benefit (cost), net change in AOCL	16	20
Prior service benefit (cost), ending balance	(71)	(87)
Prior service benefit (cost), beginning balance	22	122
Deferred income tax benefit (expense), recognition of net periodic benefits expense	(4)	(37)
Deferred income tax benefit (expense), deferrals	44	(63)
Deferred income tax benefit (expense), net change in AOCL	40	(100)
Prior service benefit (cost), ending balance	62	22
Total, beginning balance	(58)	(235)
Total, recognition of net periodic benefits expense	12	(17)
Total, deferrals	(138)	194
Total, net change in AOCL	(126)	177
Total, ending balance	$ (184)	(58)
Accounting Standards Update 2018-02 | Pension Plan
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Deferred income tax benefit (expense), recognition of net periodic benefits expense		375
Accounting Standards Update 2018-02 | Other Postretirement Benefits Plan
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Deferred income tax benefit (expense), recognition of net periodic benefits expense		$ 32