Number of purchase periods.

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- Definition

Share based compensation arrangement by share based payment award vesting period for achievement of specific milestone.

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Term of offering period.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Aggregate number of common shares reserved for future issuance.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for option under share-based payment arrangement.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of additional shares authorized for issuance under share-based payment arrangement.

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Gross number of share options (or share units) granted during the period.

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Number of options vested.

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Purchase price of common stock expressed as a percentage of its fair value.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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Aggregate change in value for stock issued during the period as a result of employee stock purchase plan.

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Stock-Based Compensation - Additional Information (Details) $ in Thousands	1 Months Ended		3 Months Ended			9 Months Ended		12 Months Ended
	Jan. 31, 2021 shares	Jan. 31, 2020 shares	Sep. 30, 2021 USD ($) shares	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) PurchasePeriod shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2019 shares	Dec. 31, 2020 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized \| $			$ 12,377		$ 8,241	$ 33,389	$ 21,213
Common stock, shares issued			51,698,775			51,698,775			51,112,302
Stock based compensation expense related to ESPP \| $				$ 305
Unrecognized compensation cost related to unvested share based awards \| $			$ 122,400			$ 122,400
Unrecognized compensation weighted-average period expected for recognition						2 years 9 months 18 days
Performance Based Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, granted								128,900
Awards vested						28,905
Performance-based Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, vested						42,536
Stock options, granted						669,581	0	170,150
Performance Based Equity Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period for achievement of specific milestone						one-quarter
Stock-based compensation expense recognized \| $			$ 2,100		$ 1,800	$ 8,300	$ 5,500
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock available for issuance increased	2,000,000.0	1,800,000
2018 Employee Share Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Issued During Period, Shares, Employee Stock Purchase Plans			0			4,295
Common stock, shares reserved for issuance			460,000			460,000
Term of offering period						12 months
Number of purchase periods \| PurchasePeriod						2
Common stock purchase price percentage						85.00%
Stock based compensation expense related to ESPP \| $			$ 100			$ 200