Stock-Based Compensation - Annual Activity Of Stock Options (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Number of Options, Balance at beginning of year	810
Number of Options - Granted	0
Number of Options - Exercised	(537)
Number of Options - Forfeited/expired	0
Number of Options, Balance at end of year	273	810
Weighted Average Exercise Price, Balance at beginning of year	$ 2.91
Weighted Average Exercise Price - Granted	0.00
Weighted Average Exercise Price - Exercised	0.79
Weighted Average Exercise Price - Forfeited/expired	0.00
Weighted Average Exercise Price, Balance at end of year	$ 7.08	$ 2.91
Aggregate Intrinsic Value - Beginning of Year	$ 27
Aggregate Intrinsic Value - Granted	$ 0
Aggregate Intrinsic Value - Exercised	$ 21	$ 1	$ 1
Aggregate Intrinsic Value - End of Year	$ 10	$ 27
Weighted Average Remaining Contractual Term	1 year 8 months	2 years 3 months

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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The grant-date intrinsic value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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-Publisher FASB
-Name Accounting Standards Codification
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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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