Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period			$ 4,725	$ 4,142
Amounts reclassified from accumulated other comprehensive loss	$ 34	$ 17	70	59
Balance at end of period	5,121	4,704	5,121	4,704
Foreign Currency Translation Adjustment
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period			(1,155)	(946)
Other comprehensive income (loss) before reclassifications			155	(7)
Amounts reclassified from accumulated other comprehensive loss			0	0
Balance at end of period	(1,000)	(953)	(1,000)	(953)
Unrecognized Net Actuarial Losses and Prior Service Costs
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period			(3,053)	(3,071)
Other comprehensive income (loss) before reclassifications			(23)	2
Amounts reclassified from accumulated other comprehensive loss	33	17	72	64
Balance at end of period	(3,004)	(3,005)	(3,004)	(3,005)
Deferred Derivative Gains (Losses)
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period			10	7
Other comprehensive income (loss) before reclassifications			(19)	(1)
Amounts reclassified from accumulated other comprehensive loss			(2)	(5)
Balance at end of period	(11)	1	(11)	1
Total
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period			(4,198)	(4,010)
Other comprehensive income (loss) before reclassifications			113	(6)
Amounts reclassified from accumulated other comprehensive loss			70	59
Balance at end of period	$ (4,015)	$ (3,957)	$ (4,015)	$ (3,957)