Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	$ 4,850	$ 4,725
Balance at end of period	4,844	5,145
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	(133)	121
Amounts reclassified from accumulated other comprehensive loss, net of tax	0	0
Balance at end of period	(1,048)	(1,034)
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	19	3
Amounts reclassified from accumulated other comprehensive loss, net of tax	57	39
Balance at end of period	(2,976)	(3,011)
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	(14)
Amounts reclassified from accumulated other comprehensive loss, net of tax	5	(3)
Balance at end of period	2	(7)
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	(108)	110
Amounts reclassified from accumulated other comprehensive loss, net of tax	62	36
Balance at end of period	$ (4,022)	$ (4,052)