Stock-based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures [Abstract]
Stock Options, Outstanding, Beginning Balance \| shares	6,689,311
Stock Options, Granted \| shares	2,136,873
Stock Options, Exercised \| shares	(475,062)
Stock Options, Cancelled or forfeited \| shares	(337,020)
Stock Options, Outstanding, Ending Balance \| shares	8,014,102	6,689,311
Stock Options, Exercisable \| shares	3,544,643
Stock Options, Vested or expected to vest \| shares	8,014,102
Weighted-Average Exercise Price, Outstanding, Beginning Balance \| $ / shares	$ 25.42
Weighted-Average Exercise Price, Granted \| $ / shares	37.23
Weighted-Average Exercise Price, Exercised \| $ / shares	12.03
Weighted-Average Exercise Price, Cancelled or forfeited \| $ / shares	31.50
Weighted-Average Exercise Price, Outstanding, Ending Balance \| $ / shares	29.10	$ 25.42
Weighted-Average Exercise Price, Exercisable \| $ / shares	21.65
Weighted-Average Exercise Price, Vested or expected to vest \| $ / shares	$ 29.10
Weighted-Average Remaining Contractual Term	8 years	8 years 3 months 18 days
Weighted-Average Remaining Contractual Term, Exercisable	7 years 1 month 6 days
Weighted-Average Remaining Contractual Term, Vested or expected to vest	8 years
Aggregate Intrinsic Value, Outstanding Balance \| $	$ 119,011	$ 68,572
Aggregate Intrinsic Value, Exercisable \| $	76,945
Aggregate Intrinsic Value, Vested or expected to vest \| $	$ 119,011

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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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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 exercise price, at which grantee can acquire shares reserved for issuance, for 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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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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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. 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 share options (or share units) exercised during the current period.

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