Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of options, Outstanding beginning balance \| shares	5,559,545
Number of options, Granted \| shares	3,916,982
Number of options, Exercised \| shares	(205,972)
Number of options, Cancelled or forfeited \| shares	(1,271,173)
Number of options, Outstanding ending balance \| shares	7,999,382	5,559,545
Number of options, Exercisable \| shares	2,812,685
Weighted average exercise price, Outstanding beginning balance \| $ / shares	$ 14.97
Weighted average exercise price, Granted \| $ / shares	13.25
Weighted average exercise price, Exercised \| $ / shares	4.23
Weighted average exercise price, Cancelled or forfeited \| $ / shares	16.44
Weighted average exercise price, Outstanding ending balance \| $ / shares	14.20	$ 14.97
Weighted average exercise price, Exercisable \| $ / shares	$ 13.45
Weighted average remaining contractual term, Outstanding balance	8 years 18 days	7 years 7 months 24 days
Weighted average remaining contractual term, Exercisable	6 years 2 months 1 day
Aggregate intrinsic value, Outstanding balance \| $	$ 2,634	$ 13,511
Aggregate intrinsic value, Exercisable \| $	$ 2,593

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

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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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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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