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Employee Benefit Plans and Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	May 28, 2015
Schedule Of Stock Options [Line Items]
Shares reserved for issuance		4.0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Stock options outstanding at beginning of period, number of shares	0.4
Stock options outstanding at beginning of period	$ 6.71
Exercised, number of shares	(0.3)
Exercised	$ 6.34
Forfeited, number of shares	0.0
Forfeited	$ 0.00
Stock options outstanding at end of period, number of shares	0.1
Stock options outstanding at end of period	$ 8.02
Stock options exercisable at end of period, number of shares	0.1
Stock options exercisable at end of period	$ 8.02
Stock options outstanding, weighted-average remaining contractual term (years)	11 months 12 days
Stock options exercisable, weighted-average remaining contractual term (years)	11 months 12 days
Stock options outstanding, aggregate intrinsic value	$ 9.6
Stock options exercisable, aggregate intrinsic value	$ 9.6
2015 Employee Stock Purchase Plan
Schedule Of Stock Options [Line Items]
Shares reserved for issuance	1.1

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Schedule Of Stock Options [Line Items]

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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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.

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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.

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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.

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

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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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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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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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