Employee Benefit Plans - Summary of Stock Option Activity (Detail) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) $ / shares shares
Schedule Of Stock Options [Line Items]
Outstanding at December 31, 2016, number of shares \| shares	0.7
Exercised, number of shares \| shares	(0.3)
Forfeited, number of shares \| shares	0.0
Outstanding at December 31, 2017, number of shares \| shares	0.4
Exercisable at December 31, 2017, number of shares \| shares	0.4
Outstanding at December 31, 2016 \| $ / shares	$ 7.40
Exercised \| $ / shares	8.21
Forfeited \| $ / shares	0.00
Outstanding at December 31, 2017 \| $ / shares	6.71
Exercisable at December 31, 2017 \| $ / shares	$ 6.71
Outstanding, Weighted-Average Remaining Contractual Term (years)	1 year 3 months 18 days
Exercisable, Weighted-Average Remaining Contractual Term (years)	1 year 3 months 18 days
Outstanding, Aggregate Intrinsic Value \| $	$ 19.5
Exercisable, Aggregate Intrinsic Value \| $	$ 19.5

Schedule Of Stock Options [Line Items]

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- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
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- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

+ References

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- Definition

For presentations that combine terminations, 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 or that expired.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 2
-Subparagraph (c)(1)(iv)(3)-(4)
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- Definition

Weighted average price of options that were either forfeited or expired.

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- Definition

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

+ References

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- Definition

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

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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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- Definition

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

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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-Name Accounting Standards Codification
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