Accumulated Other Comprehensive Loss (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 19,828,000,000
Other comprehensive loss before reclassifications			(46,000,000)	$ (160,000,000)
Amounts reclassified from accumulated other comprehensive loss			88,000,000	71,000,000
Other comprehensive income (loss)	$ 10,000,000	$ (203,000,000)	42,000,000	(89,000,000)
Balance at end of period	13,692,000,000	22,988,000,000	13,692,000,000	22,988,000,000
Defined benefit plan | Pension plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(2,173,000,000)	(1,731,000,000)
Other comprehensive loss before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive loss			82,000,000	64,000,000
Other comprehensive income (loss)			82,000,000	64,000,000
Balance at end of period	(2,091,000,000)	(2,042,000,000)	(2,091,000,000)	(2,042,000,000)
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					$ (375,000,000)
Defined benefit plan | Post-retirement benefit plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(58,000,000)	(235,000,000)
Other comprehensive loss before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive loss			6,000,000	7,000,000
Other comprehensive income (loss)			6,000,000	7,000,000
Balance at end of period	(52,000,000)	(260,000,000)	(52,000,000)	(260,000,000)
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					(32,000,000)
Foreign currency translation adjustment and other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(230,000,000)	(29,000,000)
Other comprehensive loss before reclassifications			(3,000,000)	(160,000,000)
Amounts reclassified from accumulated other comprehensive loss			0	0
Other comprehensive income (loss)			(3,000,000)	(160,000,000)
Balance at end of period	(233,000,000)	(189,000,000)	(233,000,000)	(189,000,000)
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			(43,000,000)
Amounts reclassified from accumulated other comprehensive loss			0
Other comprehensive income (loss)			(43,000,000)
Balance at end of period	(43,000,000)		(43,000,000)
Accumulated other comprehensive income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,429,000,000)	(2,288,000,000)	(2,461,000,000)	(1,995,000,000)
Other comprehensive income (loss)	10,000,000	(203,000,000)	42,000,000	(89,000,000)
Balance at end of period	$ (2,419,000,000)	$ (2,491,000,000)	$ (2,419,000,000)	$ (2,491,000,000)
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					$ (407,000,000)