The amount of cash investment.

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The amount of incremental stock-based compensation expense.

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Number of liquidity shares.

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No definition available.

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- Definition

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

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- Definition

Aggregate number of common shares reserved for future issuance.

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- Definition

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

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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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- Definition

Amount of tax benefit from exercise of option under share-based payment arrangement.

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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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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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No definition available.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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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 ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
Stockholders' Equity - (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 28, 2021	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
Income tax provision (benefit)	$ 0	$ 0	$ 0	$ 0
Sale of transaction
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Liquidity of Shares	8,043,830		8,043,830
Other events
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental stock-based compensation expense			$ 13,500
Liquidity of Shares	9,124,000		9,124,000
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 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 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 authorized (in shares)	25,000,000		25,000,000		25,000,000	25,000,000
Verizon
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash investment								$ 35,000
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
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