Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 19,828
Other comprehensive income (loss) before reclassifications	(12)	$ 79
Amounts reclassified from accumulated other comprehensive income	44	35
Other comprehensive income	32	114
Balance at end of period	13,543	23,443
Defined benefit plan | Pension plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,173)	(1,731)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	41	31
Other comprehensive income	41	31
Balance at end of period	(2,132)	(2,075)
Defined benefit plan | Pension plans | Accounting Standards Update 2018-02
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			$ (375)
Defined benefit plan | Post-retirement benefit plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(58)	(235)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	3	4
Other comprehensive income	3	4
Balance at end of period	(55)	(263)
Defined benefit plan | Post-retirement benefit plans | Accounting Standards Update 2018-02
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			(32)
Foreign currency translation adjustment and other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(230)	(29)
Other comprehensive income (loss) before reclassifications	5	79
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income	5	79
Balance at end of period	(225)	50
Foreign currency translation adjustment and other | Accounting Standards Update 2018-02
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			0
Interest rate hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	0
Other comprehensive income (loss) before reclassifications	(17)
Amounts reclassified from accumulated other comprehensive income	0
Other comprehensive income	(17)
Balance at end of period	(17)
Accumulated other comprehensive income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,461)	(1,995)
Other comprehensive income	32	114
Balance at end of period	$ (2,429)	$ (2,288)
Accumulated other comprehensive income | Accounting Standards Update 2018-02
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			$ (407)