Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	$ 4,850	$ 4,725
Balance at end of period	5,000	5,121
Foreign Currency Translation Adjustment
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	(915)	(1,155)
Other comprehensive income (loss) before reclassifications, net of tax	(210)	155
Amounts reclassified from accumulated other comprehensive loss, net of tax	0	0
Balance at end of period	(1,125)	(1,000)
Unrecognized Net Actuarial Losses and Prior Service Costs
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	(3,052)	(3,053)
Other comprehensive income (loss) before reclassifications, net of tax	(1)	(23)
Amounts reclassified from accumulated other comprehensive loss, net of tax	92	72
Balance at end of period	(2,961)	(3,004)
Deferred Derivative Gains (Losses)
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	(9)	10
Other comprehensive income (loss) before reclassifications, net of tax	6	(19)
Amounts reclassified from accumulated other comprehensive loss, net of tax	6	(2)
Balance at end of period	3	(11)
Total
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	(3,976)	(4,198)
Other comprehensive income (loss) before reclassifications, net of tax	(205)	113
Amounts reclassified from accumulated other comprehensive loss, net of tax	98	70
Balance at end of period	$ (4,083)	$ (4,015)