Reclassifications out of Accumulated Other Comprehensive Loss - Changes in AOCL (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 4,603
Balance at end of period	4,864	$ 4,603
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(915)	(1,155)	$ (946)
Other comprehensive income (loss) before reclassifications	(245)	240	(209)
Amounts reclassified from accumulated other comprehensive loss	0	0	0
Balance at end of period	(1,160)	(915)	(1,155)
Unrecognized Net Actuarial Losses and Prior Service Costs
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(3,052)	(3,053)	(3,071)
Other comprehensive income (loss) before reclassifications	4	(103)	(62)
Amounts reclassified from accumulated other comprehensive loss	125	104	80
Balance at end of period	(2,923)	(3,052)	(3,053)
Deferred Derivative Gains (Losses)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(9)	10	7
Other comprehensive income (loss) before reclassifications	9	(20)	8
Amounts reclassified from accumulated other comprehensive loss	7	1	(5)
Balance at end of period	7	(9)	10
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(3,976)	(4,198)	(4,010)
Other comprehensive income (loss) before reclassifications	(232)	117	(263)
Amounts reclassified from accumulated other comprehensive loss	132	105	75
Balance at end of period	$ (4,076)	$ (3,976)	$ (4,198)