Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	$ (2,172)
Intraperiod tax allocation		$ (8)
Other comprehensive income (loss)	351	(159)	$ (539)
Balance at end of the period	(1,826)	(2,172)
Net Effect of Exchange Rate Fluctuations
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(788)	(568)
Change before reclassifications	65	(220)
Other comprehensive income (loss)	65	(220)
Balance at end of the period	(723)	(788)	(568)
Change in Certain Derivative Instruments
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(4)	(17)
Change before reclassifications	(3)	7
Amounts reclassified from accumulated other comprehensive income	(6)	6
Tax effect	1
Other comprehensive income (loss)	(8)	13
Balance at end of the period	(12)	(4)	(17)
Employee Benefit Plans
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(1,380)	(1,432)
Change before reclassifications	(8)	(2)
Amounts reclassified from accumulated other comprehensive income	285	72
Translation effect	32	(25)
Tax effect	(20)	15
Intraperiod tax allocation		(8)
Other comprehensive income (loss)	289	52
Balance at end of the period	(1,091)	(1,380)	(1,432)
Accumulated Other Comprehensive Loss.
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(2,172)	(2,017)
Change before reclassifications	54	(215)
Amounts reclassified from accumulated other comprehensive income	279	78
Translation effect	32	(25)
Tax effect	(19)	15
Intraperiod tax allocation		(8)
Other comprehensive income (loss)	346	(155)
Balance at end of the period	$ (1,826)	$ (2,172)	$ (2,017)