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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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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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 weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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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Stockholders' Equity - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 16, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted term					10 years
Share-based compensation costs	$ 503	$ 327	$ 850	$ 988	$ 1,189	$ 2,813	$ 5,020
2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period					4 years
Class A Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares, shares issued (in shares)	12,803,162		12,803,162		5,034,459	5,014,300
Common shares, shares outstanding	5,081,771		5,081,771		5,034,459	5,014,300
Common shares, shares authorized (in shares)	500,000,000		500,000,000		500,000,000	500,000,000
Class B Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares, shares issued (in shares)	27,391,338		27,391,338		34,362,554	34,152,328
Common shares, shares outstanding	27,143,111		27,143,111		34,114,327	33,904,101
Common shares, shares authorized (in shares)	500,000,000		500,000,000		500,000,000	500,000,000
Class C Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares, shares issued (in shares)	21,170,037		21,170,037		0	0
Common shares, shares outstanding	21,170,037		21,170,037		0	0
Common shares, shares authorized (in shares)	25,000,000		25,000,000		25,000,000	25,000,000
Stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation costs			$ 2,600		$ 10,500
Weighted-average remaining requisite service period			3 years		2 months 12 days
Weighted average fair value of stock options granted					$ 1.03	$ 1.01	$ 1.26
Intrinsic value of stock options exercised					$ 400	$ 1,600	$ 2,000
Stock option \| 2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant					16,895,765
Number of shares available for future issuance					17,327,629	14,266,318	14,000,528	15,700,000