Reclassifications out of Accumulated Other Comprehensive Loss - Changes in AOCL (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 4,545	$ 5,070
Ending balance	3,697	5,031
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	(216)	30
Amounts reclassified from accumulated other comprehensive loss, net of tax	0	0
Ending balance	(1,372)	(1,130)
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	(1)	4
Amounts reclassified from accumulated other comprehensive loss, net of tax	26	26
Ending balance	(2,958)	(2,893)
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	18	5
Amounts reclassified from accumulated other comprehensive loss, net of tax	(4)	(3)
Ending balance	17	9
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	(199)	39
Amounts reclassified from accumulated other comprehensive loss, net of tax	22	23
Ending balance	$ (4,313)	$ (4,014)