Assets and Liabilities Adjusted to Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Money Market Funds \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure, recurring	$ 0	$ 135

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

No definition available.

Name:	us-gaap_AssetsFairValueDisclosureRecurring
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant