Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Other comprehensive income (loss)	$ 46	$ 20	$ (66)	$ 227
Net Effect of Exchange Rate Fluctuations
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(856)	(578)	(724)	(788)
Change before reclassifications	40	9	(92)	219
Other comprehensive income (loss)	40	9	(92)	219
Balance at end of the period	(816)	(569)	(816)	(569)
Change in Certain Derivative Instruments
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(18)	(14)	(12)	(4)
Change before reclassifications	(11)		(7)
Amounts reclassified from accumulated other comprehensive income	9			(10)
Translation effect	1
Other comprehensive income (loss)	(1)		(7)	(10)
Balance at end of the period	(19)	(14)	(19)	(14)
Employee Benefit Plans
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(1,055)	(1,365)	(1,091)	(1,380)
Change before reclassifications	(1)	(2)	(8)	(12)
Amounts reclassified from accumulated other comprehensive income	16	19	49	58
Translation effect		(7)	10	(21)
Other comprehensive income (loss)	15	10	51	25
Balance at end of the period	(1,040)	(1,355)	(1,040)	(1,355)
Accumulated Other Comprehensive Loss.
Increase (Decrease) Accumulated Other Comprehensive Loss, Net of Tax
Balance at beginning of the period	(1,929)	(1,957)	(1,827)	(2,172)
Change before reclassifications	28	7	(107)	207
Amounts reclassified from accumulated other comprehensive income	25	19	49	48
Translation effect	1	(7)	10	(21)
Other comprehensive income (loss)	54	19	(48)	234
Balance at end of the period	$ (1,875)	$ (1,938)	$ (1,875)	$ (1,938)