Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Options
Options, Beginning Balance	2,705,849
Options, Granted	584,305	618,899
Options, Forfeited	(115,035)
Options, Exercised	(487,317)
Options, Ending Balance	2,687,802		2,705,849
Options, Vested and expected to vest	2,682,769
Options, Exercisable	1,415,269
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Beginning Balance	$ 25.53
Weighted-Average Exercise Price, Granted	68.74
Weighted-Average Exercise Price, Forfeited	34.21
Weighted-Average Exercise Price, Exercised	26.70
Weighted-Average Exercise Price, Ending Balance	34.34		$ 25.53
Weighted-Average Exercise Price, Vested and expected to vest	34.28
Weighted-Average Exercise Price, Exercisable	$ 21.73
Aggregate Intrinsic Value/Weighted-Average Exercise Term
Aggregate Intrinsic Value	$ 280,279		$ 125,197
Aggregate Intrinsic Value, Vested and expected to vest	279,912
Aggregate Intrinsic Value, Exercisable	$ 165,427
Weighted-Average Exercise Term, Outstanding Balance	7 years 1 month 9 days		7 years 3 months 10 days
Weighted-Average Exercise Term, Vested and expected to vest	7 years 1 month 6 days
Weighted-Average Exercise Term, Exercisable	6 years 1 month 6 days

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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 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.

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

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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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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