Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 19,828
Other comprehensive loss before reclassifications			(169)	$ (161)
Amounts reclassified from accumulated other comprehensive loss			131	108
Other comprehensive (loss) income	$ (80)	$ 36	(38)	(53)
Balance at end of period	13,674	22,802	13,674	22,802
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 loss before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive loss			122	97
Other comprehensive (loss) income			122	97
Balance at end of period	(2,051)	(2,009)	(2,051)	(2,009)
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 loss before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive loss			9	11
Other comprehensive (loss) income			9	11
Balance at end of period	(49)	(256)	(49)	(256)
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 loss before reclassifications			(115)	(161)
Amounts reclassified from accumulated other comprehensive loss			0	0
Other comprehensive (loss) income			(115)	(161)
Balance at end of period	(345)	(190)	(345)	(190)
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 loss before reclassifications			(54)
Amounts reclassified from accumulated other comprehensive loss			0
Other comprehensive (loss) income			(54)
Balance at end of period	(54)		(54)
Accumulated other comprehensive income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,419)	(2,491)	(2,461)	(1,995)
Other comprehensive (loss) income	(80)	36	(38)	(53)
Balance at end of period	$ (2,499)	$ (2,455)	$ (2,499)	$ (2,455)
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)