Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Excess tax benefits from share-based payments	$ 113	$ 81	$ 0

Amount of excess tax benefits recorded as a component of income tax expense.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration