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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis and Subject to Fair Value Disclosure Requirements (Detail) - Recurring - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Money market funds at December 31, 2017	$ 501	$ 473
Quoted Price as in Active Markets for Identical Assets (Level 1)
Assets
Money market funds at December 31, 2017	501	473
Significant Other Observable Inputs (Level 2)
Assets
Money market funds at December 31, 2017	0	0
Significant Unobservable Inputs (Level 3)
Assets
Money market funds at December 31, 2017	$ 0	$ 0

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- Definition

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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