Income Taxes - Unrecognized Tax Benefits - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Unrecognized tax balance	$ 487	$ 458	$ 440	$ 367
Unrecognized tax benefits that would impact effective tax rate	420
Interest and penalties, net of tax benefits, in uncertain tax positions	3
Unrecognized tax benefits, interest and penalties accrued	43	$ 54
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Income Tax Contingency [Line Items]
Unrecognized tax balance	100
Unrecognized tax benefits that would impact effective tax rate	$ 95