Employee Benefits - Amounts Recognized in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Total, beginning balance	$ (1,966)	$ (2,057)
Total, recognition of net periodic benefits expense	(264)	138
Total, deferrals	(1)	(47)
Total, net change in AOCL	(265)	91
Total, ending balance	(2,231)	(1,966)
Pension Plan
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Net actuarial (loss) gain, beginning balance	(2,892)	(3,148)
Net actuarial (loss) gain, recognition of net periodic benefits expense	179	205
Net actuarial (loss) gain, deferrals	(260)	51
Net actuarial (loss) gain, net change in AOCL	(81)	256
Net actuarial (loss) gain, ending balance	(2,973)	(2,892)
Prior service benefit (cost), beginning balance	54	62
Prior service benefit (cost), recognition of net periodic benefits expense	(8)	(8)
Prior service benefit (cost), deferrals	0	0
Prior service benefit (cost), net change in AOCL	(8)	(8)
Prior service benefit (cost), ending balance	46	54
Prior service benefit (cost), beginning balance	1,107	1,191
Deferred income tax benefit (expense), recognition of net periodic benefits expense	(418)	(72)
Deferred income tax benefit (expense), deferrals	65	(12)
Deferred income tax benefit (expense), net change in AOCL	(353)	(84)
Prior service benefit (cost), ending balance	754	1,107
Total, beginning balance	(1,731)	(1,895)
Total, recognition of net periodic benefits expense	(247)	125
Total, deferrals	(195)	39
Total, net change in AOCL	(442)	164
Total, ending balance	(2,173)	(1,731)
Pension Plan | Accounting Standards Update 2018-02
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Deferred income tax benefit (expense), recognition of net periodic benefits expense	(375)
Other Postretirement Benefits Plan
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Net actuarial (loss) gain, beginning balance	(250)	(137)
Net actuarial (loss) gain, recognition of net periodic benefits expense	0	0
Net actuarial (loss) gain, deferrals	257	(113)
Net actuarial (loss) gain, net change in AOCL	257	(113)
Net actuarial (loss) gain, ending balance	7	(250)
Prior service benefit (cost), beginning balance	(107)	(127)
Prior service benefit (cost), recognition of net periodic benefits expense	20	20
Prior service benefit (cost), deferrals	0	0
Prior service benefit (cost), net change in AOCL	20	20
Prior service benefit (cost), ending balance	(87)	(107)
Prior service benefit (cost), beginning balance	122	102
Deferred income tax benefit (expense), recognition of net periodic benefits expense	(37)	(7)
Deferred income tax benefit (expense), deferrals	(63)	27
Deferred income tax benefit (expense), net change in AOCL	(100)	20
Prior service benefit (cost), ending balance	22	122
Total, beginning balance	(235)	(162)
Total, recognition of net periodic benefits expense	(17)	13
Total, deferrals	194	(86)
Total, net change in AOCL	177	(73)
Total, ending balance	(58)	$ (235)
Other Postretirement Benefits Plan | Accounting Standards Update 2018-02
Defined Benefit Plan, Net Change in Accumulated Other Comprehensive Income [Roll Forward]
Deferred income tax benefit (expense), recognition of net periodic benefits expense	$ (32)