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Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized, Annual Increase, Percent Of Shares Outstanding

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Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized, Maximum Number Of Shares Issued Upon Exercise

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Share-Based Compensation Arrangement By Share-Based Payment Award, Number Of Shares Available For Grant, Annual Increase

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Stock Issued During Period, Shares, Exercise of Stock Option Upon Repayment

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Stock Issued During Period, Value, Exercise of Stock Option Upon Repayment

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Stock Purchase Agreement, Number Of Executive Employees

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Subscriptions Receivable, Weighted Average Interest Rate

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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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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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Cash inflow from repayment of loans for purchasing common stock.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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 equity instruments other than options outstanding, including both vested and non-vested instruments.

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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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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Note received instead of cash as contribution to equity. The transaction may be a sale of capital stock or a contribution to paid-in capital.

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Stock-based Compensation - Additional Information (Details) $ in Millions			3 Months Ended		9 Months Ended		12 Months Ended			24 Months Ended
	Jul. 02, 2020 shares	Jun. 08, 2020 USD ($) employee	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) employee	Jan. 01, 2021	Jul. 31, 2015 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $			$ 12.7	$ 2.7	$ 30.7	$ 7.3	$ 10.6	$ 4.3	$ 2.1
Proceeds from release of shares upon repayment \| $					$ 0.0	1.3	1.3	0.0	$ 0.0
Executives
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Release of shares upon repayment \| $							$ 0.1
Release of shares upon repayment (shares)							105,487
Settlement of subscriptions receivable, number of executive employees \| employee		2
Incentive Compensation Plan 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (shares)	5,503,678
Common stock reserved for issuance, annual increase, percentage of outstanding shares	5.00%										5.00%
Maximum shares that may be issued upon exercise of incentive stock options (shares)	3,650,000
Common stock available for grant (shares)			5,733,902		5,733,902		4,936,501
Employee Stock Purchase Plan 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (shares)	1,000,000
Common stock reserved for issuance, annual increase, percentage of outstanding shares	1.00%										1.00%
Common stock available for grant (shares)			0		0
Number of shares available for issuance, annual increase (shares)	1,000,000
Incentive Share Option Plan 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (shares)												7,312,590
Common stock available for grant (shares)			0		0		0
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $			$ 12.0	$ 2.7	$ 28.0	$ 7.3
Unrecognized expense \| $			$ 104.7		$ 104.7		$ 36.9
Unrecognized expense, period for recognition					1 year 4 months 24 days		1 year 6 months
Share-based Payment Arrangement, Option [Member] \| Executives
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $							$ 0.2	$ 0.2
Unrecognized expense \| $							$ 0.1
Unrecognized expense, period for recognition							3 months 18 days
Subscriptions receivable, number of executive employees \| employee										2
Subscriptions receivable \| $										$ 1.5
Subscriptions receivable, weighted average interest rate										1.90%
Proceeds from release of shares upon repayment \| $		$ 1.3
Share-based Payment Arrangement, Option [Member] \| Incentive Share Option Plan 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years		4 years
Expiration period					10 years		10 years
Restricted stock \| Executives
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards outstanding (shares)								513,537