Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 17	$ 26	36	39
Balance at end of period	5,145	4,408	5,145	4,408
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			121	5
Amounts reclassified from accumulated other comprehensive loss			0	0
Balance at end of period	(1,034)	(941)	(1,034)	(941)
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			3	1
Amounts reclassified from accumulated other comprehensive loss	19	31	39	47
Balance at end of period	(3,011)	(3,023)	(3,011)	(3,023)
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			(14)	3
Amounts reclassified from accumulated other comprehensive loss			(3)	(8)
Balance at end of period	(7)	2	(7)	2
Total
Changes in Accumulated Other Comprehensive Loss [Roll Forward]
Balance at beginning of period			(4,198)	(4,010)
Other comprehensive income (loss) before reclassifications			110	9
Amounts reclassified from accumulated other comprehensive loss			36	39
Balance at end of period	$ (4,052)	$ (3,962)	$ (4,052)	$ (3,962)