Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	$ 4,850	$ 4,725
Balance at end of period	4,962	4,960
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	80	78
Amounts reclassified from accumulated other comprehensive loss, net of tax	0	0
Balance at end of period	(835)	(1,077)
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	3	4
Amounts reclassified from accumulated other comprehensive loss, net of tax	27	20
Balance at end of period	(3,022)	(3,029)
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	(4)	(6)
Amounts reclassified from accumulated other comprehensive loss, net of tax	3	(1)
Balance at end of period	(10)	3
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	79	76
Amounts reclassified from accumulated other comprehensive loss, net of tax	30	19
Balance at end of period	$ (3,867)	$ (4,103)