Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period	$ 5,070	$ 4,850
Balance at end of period	5,031	4,962
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	30	80
Amounts reclassified from accumulated other comprehensive loss, net of tax	0	0
Balance at end of period	(1,130)	(835)
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	4	3
Amounts reclassified from accumulated other comprehensive loss, net of tax	26	27
Balance at end of period	(2,893)	(3,022)
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	5	(4)
Amounts reclassified from accumulated other comprehensive loss, net of tax	(3)	3
Balance at end of period	9	(10)
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	39	79
Amounts reclassified from accumulated other comprehensive loss, net of tax	23	30
Balance at end of period	$ (4,014)	$ (3,867)