Stock-Based Compensation - Summary of Company's Stock Option Activity and Related Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Shares Available for Grant
Shares Available for Grant, Beginning balance	969,783
Shares Available for Grant, Additional options authorized	8,326,158
Shares Available for Grant, Options granted	(5,716,505)
Shares Available for Grant, Options canceled	801,933
Shares Available for Grant, Ending balance	2,613,827	969,783
Shares Available for Grant, RSUs granted	(1,767,542)
Number of Stock Options Outstanding
Number of Stock Options Outstanding, Beginning balance	8,592,747
Number of Stock Options Outstanding, Options granted	5,716,505
Number of Stock Options Outstanding, Options exercised	(1,607,108)
Number of Stock Options Outstanding, Options canceled	(801,933)
Number of Stock Options Outstanding, Ending balance	11,900,211	8,592,747
Number of Stock Options Outstanding, Vested and exercisable	4,731,731
Number of Stock Options Outstanding, Vested and expected to vest	11,397,852
Weighted- Average Exercise Price
Weighted- Average Exercise Price, Beginning balance	$ 3.69
Weighted- Average Exercise Price, Options granted	8.66
Weighted- Average Exercise Price, Options exercised	2.22
Weighted- Average Exercise Price, Options canceled	6.00
Weighted- Average Exercise Price, Ending balance	6.12	$ 3.69
Weighted- Average Exercise Price, Vested and exercisable	3.57
Weighted- Average Exercise Price, Vested and expected to vest	$ 6.06
Weighted- Average Remaining Contractual Life (Years)
Options Outstanding, Weighted- Average Remaining Contractual Life (Years)	7 years 3 months 18 days	7 years 4 months 24 days
Weighted- Average Remaining Contractual Life (Years), Vested and exercisable	5 years 1 month 6 days
Weighted- Average Remaining Contractual Life (Years), Vested and expected to vest	7 years 2 months 12 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value	$ 39,509	$ 47,864
Aggregate Intrinsic Value, Vested and exercisable	27,404
Aggregate Intrinsic Value, Vested and expected to vest	$ 38,576

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Number of additional shares authorized for issuance under an established share-based compensation plan.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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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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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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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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Number of fully vested and expected to vest exercisable options that may 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 exercisable or convertible options. 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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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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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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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 exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the 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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Share-based compensation arrangement by share-based payment award number of shares available for grant.

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Share-based compensation arrangement by share-based payment award options weighted average remaining contractual term.

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