Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Loss [Line Items]
Balance at beginning of period	$ 19,272
Other comprehensive income (loss) before reclassifications, net of tax	72	$ 20
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) income	78	21
Balance at end of period	19,020
Pension Plans
Accumulated Other Comprehensive Loss [Line Items]
Balance at beginning of period		(34)
Other comprehensive income (loss) before reclassifications, net of tax		0
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) income		1
Balance at end of period		(33)
Foreign Currency Translation Adjustments and Other
Accumulated Other Comprehensive Loss [Line Items]
Balance at beginning of period	18	(353)
Other comprehensive income (loss) before reclassifications, net of tax	72	20
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) income	78	20
Balance at end of period	96	(333)
AOCI Attributable to Parent
Accumulated Other Comprehensive Loss [Line Items]
Balance at beginning of period	18	(387)
Balance at end of period	$ 96	$ (366)