Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 13,470
Other comprehensive loss before reclassifications			(268)	$ (169)
Amounts reclassified from accumulated other comprehensive loss			153	131
Other comprehensive income (loss)	$ 107	$ (80)	(115)	(38)
Balance at end of period	13,689	13,674	13,689	13,674
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			110	122
Other comprehensive income (loss)			110	122
Balance at end of period	(2,119)	(2,051)	(2,119)	(2,051)
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			12	9
Other comprehensive income (loss)			12	9
Balance at end of period	(172)	(49)	(172)	(49)
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			(181)	(115)
Amounts reclassified from accumulated other comprehensive loss			0	0
Other comprehensive income (loss)			(181)	(115)
Balance at end of period	(409)	(345)	(409)	(345)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(39)	0
Other comprehensive loss before reclassifications			(87)	(54)
Amounts reclassified from accumulated other comprehensive loss			31	0
Other comprehensive income (loss)			(56)	(54)
Balance at end of period	(95)	(54)	(95)	(54)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,902)	(2,419)	(2,680)	(2,461)
Other comprehensive income (loss)	107	(80)	(115)	(38)
Balance at end of period	$ (2,795)	$ (2,499)	$ (2,795)	$ (2,499)