Stock-based Compensation - Stock Options Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Number of Options
Outstanding (shares) \| shares	4,944,711
Granted (shares) \| shares	2,319,085
Exercised (shares) \| shares	(810,575)
Cancelled (shares) \| shares	(420,423)
Outstanding (shares) \| shares	6,032,798	4,944,711
Exercisable (shares) \| shares	1,953,806
Unvested (shares) \| shares	4,078,992
Weighted- Average Exercise Price
Outstanding (usd per share) \| $ / shares	$ 20.50
Granted (usd per share) \| $ / shares	91.13
Exercised (usd per share) \| $ / shares	10.68
Cancelled (usd per share) \| $ / shares	49.50
Outstanding (usd per share) \| $ / shares	46.58	$ 20.50
Exercisable, weighted-average exercise price (usd per share) \| $ / shares	15.52
Unvested, weighted-average exercise price (usd per share) \| $ / shares	$ 61.45
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, weighted-average remaining contractual, term	8 years 4 months 17 days	8 years 3 months 18 days
Exercisable, weighted-average remaining contractual, term	7 years 2 months 15 days
Unvested, weighted-average remaining contractual, term	8 years 11 months 8 days
Outstanding, aggregate intrinsic value, outstanding \| $	$ 177,060	$ 506,580
Exercisable, aggregate intrinsic value \| $	101,420
Unvested, aggregate intrinsic value \| $	$ 75,640

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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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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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Amount by which current fair value of underlying stock exceeds exercise price of 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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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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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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