Reclassifications out of Accumulated Other Comprehensive Loss - Changes in AOCL (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 4,545	$ 5,070
Ending balance	3,013	5,049
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,156)	(1,160)
Other comprehensive income (loss) before reclassifications, net of tax	(223)	11
Amounts reclassified from accumulated other comprehensive loss, net of tax	0	0
Ending balance	(1,379)	(1,149)
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)
Other comprehensive income (loss) before reclassifications, net of tax	(9)	12
Amounts reclassified from accumulated other comprehensive loss, net of tax	55	52
Ending balance	(2,937)	(2,859)
Deferred Derivative Gains (Losses)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	3	7
Other comprehensive income (loss) before reclassifications, net of tax	19	4
Amounts reclassified from accumulated other comprehensive loss, net of tax	(8)	(5)
Ending balance	14	6
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,136)	(4,076)
Other comprehensive income (loss) before reclassifications, net of tax	(213)	27
Amounts reclassified from accumulated other comprehensive loss, net of tax	47	47
Ending balance	$ (4,302)	$ (4,002)