Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	$ 5,070	$ 4,850
Balance at end of period	5,049	4,844
Foreign Currency Translation Adjustment
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	(1,160)	(915)
Other comprehensive income (loss) before reclassifications, net of tax	11	(133)
Amounts reclassified from accumulated other comprehensive loss, net of tax	0	0
Balance at end of period	(1,149)	(1,048)
Unrecognized Net Actuarial Losses and Prior Service Costs
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	(2,923)	(3,052)
Other comprehensive income (loss) before reclassifications, net of tax	12	19
Amounts reclassified from accumulated other comprehensive loss, net of tax	52	57
Balance at end of period	(2,859)	(2,976)
Deferred Derivative Gains (Losses)
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	7	(9)
Other comprehensive income (loss) before reclassifications, net of tax	4	6
Amounts reclassified from accumulated other comprehensive loss, net of tax	(5)	5
Balance at end of period	6	2
Total
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	(4,076)	(3,976)
Other comprehensive income (loss) before reclassifications, net of tax	27	(108)
Amounts reclassified from accumulated other comprehensive loss, net of tax	47	62
Balance at end of period	$ (4,002)	$ (4,022)