Reclassifications out of Accumulated Other Comprehensive Loss - Changes in AOCL (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 4,545	$ 5,070	$ 4,850
Ending balance	3,259	4,545	5,070
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,156)	(1,160)	(915)
Other comprehensive income (loss) before reclassifications	(128)	4	(245)
Amounts reclassified from accumulated other comprehensive loss	0	0	0
Ending balance	(1,284)	(1,156)	(1,160)
Unrecognized Net Actuarial Losses and Prior Service Costs
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(2,983)	(2,923)	(3,052)
Other comprehensive income (loss) before reclassifications	(4)	(168)	4
Amounts reclassified from accumulated other comprehensive loss	131	108	125
Ending balance	(2,856)	(2,983)	(2,923)
Deferred Derivative Gains (Losses)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	3	7	(9)
Other comprehensive income (loss) before reclassifications	15	10	9
Amounts reclassified from accumulated other comprehensive loss	(13)	(14)	7
Ending balance	5	3	7
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,136)	(4,076)	(3,976)
Other comprehensive income (loss) before reclassifications	(117)	(154)	(232)
Amounts reclassified from accumulated other comprehensive loss	118	94	132
Ending balance	$ (4,135)	$ (4,136)	$ (4,076)