Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 19,272
Other comprehensive loss before reclassifications, net of tax	(164)	$ 107
Cumulative effect of adoption of ASU 2018-02, Income Statement-Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	6	(1)
Net other comprehensive loss	(158)	106
Balance at end of period	18,172
Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(34)
Other comprehensive loss before reclassifications, net of tax		1
Cumulative effect of adoption of ASU 2018-02, Income Statement-Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income		(1)
Net other comprehensive loss		0
Balance at end of period		(34)
Foreign Currency Translation Adjustment and Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	18	(353)
Other comprehensive loss before reclassifications, net of tax	(164)	106
Cumulative effect of adoption of ASU 2018-02, Income Statement-Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	6	0
Net other comprehensive loss	(158)	106
Balance at end of period	(140)	(247)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	18	(387)
Balance at end of period	$ (140)	$ (281)