Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 13,470
Other comprehensive income (loss) before reclassifications			(318)	$ (46)
Amounts reclassified from accumulated other comprehensive loss			96	88
Other comprehensive income (loss)	$ 53	$ 10	(222)	42
Balance at end of period	13,462	13,673	13,462	13,673
Defined benefit plan | Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(2,229)	(2,173)
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive loss			74	82
Other comprehensive income (loss)			74	82
Balance at end of period	(2,155)	(2,091)	(2,155)	(2,091)
Defined benefit plan | Post-Retirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(184)	(58)
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive loss			6	6
Other comprehensive income (loss)			6	6
Balance at end of period	(178)	(52)	(178)	(52)
Foreign currency translation adjustment and other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(228)	(230)
Other comprehensive income (loss) before reclassifications			(230)	(3)
Amounts reclassified from accumulated other comprehensive loss			0	0
Other comprehensive income (loss)			(230)	(3)
Balance at end of period	(458)	(233)	(458)	(233)
Interest rate swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(39)	0
Other comprehensive income (loss) before reclassifications			(88)	(43)
Amounts reclassified from accumulated other comprehensive loss			16	0
Other comprehensive income (loss)			(72)	(43)
Balance at end of period	(111)	(43)	(111)	(43)
Accumulated other comprehensive income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,955)	(2,429)	(2,680)	(2,461)
Other comprehensive income (loss)	53	10	(222)	42
Balance at end of period	$ (2,902)	$ (2,419)	$ (2,902)	$ (2,419)