Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 13,470
Other comprehensive loss before reclassifications	(319)	$ (12)
Amounts reclassified from accumulated other comprehensive loss	44	44
Other comprehensive (loss) income	(275)	32
Balance at end of period	13,291	13,524
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 loss before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	36	41
Other comprehensive (loss) income	36	41
Balance at end of period	(2,193)	(2,132)
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 loss before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	3	3
Other comprehensive (loss) income	3	3
Balance at end of period	(181)	(55)
Foreign currency translation adjustment and other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(228)	(230)
Other comprehensive loss before reclassifications	(239)	5
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive (loss) income	(239)	5
Balance at end of period	(467)	(225)
Interest rate swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(39)	0
Other comprehensive loss before reclassifications	(80)	(17)
Amounts reclassified from accumulated other comprehensive loss	5	0
Other comprehensive (loss) income	(75)	(17)
Balance at end of period	(114)	(17)
Accumulated other comprehensive income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,680)	(2,461)
Other comprehensive (loss) income	(275)	32
Balance at end of period	$ (2,955)	$ (2,429)