Summary of Accounting Policies - Recent Accounting/Restricted Net Assets (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Excess tax benefit	$ 2
Restricted Net Assets	$ 406	$ 323

Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount

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