Reclassifications out of Accumulated Other Comprehensive Loss - Changes in AOCL (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 5,070	$ 4,850	$ 4,725
Ending balance	4,545	5,070	4,850
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,160)	(915)	(1,155)
Other comprehensive income (loss) before reclassifications	4	(245)	240
Amounts reclassified from accumulated other comprehensive loss	0	0	0
Ending balance	(1,156)	(1,160)	(915)
Unrecognized Net Actuarial Losses and Prior Service Costs
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(2,923)	(3,052)	(3,053)
Other comprehensive income (loss) before reclassifications	(168)	4	(103)
Amounts reclassified from accumulated other comprehensive loss	108	125	104
Ending balance	(2,983)	(2,923)	(3,052)
Deferred Derivative Gains (Losses)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	7	(9)	10
Other comprehensive income (loss) before reclassifications	10	9	(20)
Amounts reclassified from accumulated other comprehensive loss	(14)	7	1
Ending balance	3	7	(9)
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,076)	(3,976)	(4,198)
Other comprehensive income (loss) before reclassifications	(154)	(232)	117
Amounts reclassified from accumulated other comprehensive loss	94	132	105
Ending balance	(4,136)	$ (4,076)	$ (3,976)
United Kingdom
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment for frozen plan	$ 32