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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) shares in Thousands |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 10, 2021 USD ($) | Jun. 30, 2021 shares | Dec. 31, 2023 USD ($) sourceOfRevenue tranche reporting_unit shares | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Concentration Risk [Line Items] |  |  |  |  |
Number of sources of revenue | sourceOfRevenue |  |  | 4 |  |
Number of days to take exam |  |  | 21 days |  |
Advertising costs |  |  | $ 52,969,000 | $ 48,111,000 | $ 42,964,000
Volunteer program, one-time payment to contributors | $ 4,220,000 |  |  |  |
Volunteer program, fees paid | $ 526,000 |  |  |  |
Compensation expense |  |  | $ 95,221,000 | 73,820,000 | 40,804,000
Number of reporting units | reporting_unit |  |  | 1 |  |
Impairment |  |  | $ 0 | 0 | 0
Research and development |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Compensation expense |  |  | 45,119,000 | 26,373,000 | 9,298,000
Sales and marketing |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Compensation expense |  |  | 3,908,000 | 2,540,000 | 881,000
General and administrative |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Compensation expense |  |  | $ 46,139,000 | 44,869,000 | 30,617,000
RSUs outstanding |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Number of vesting requirements | tranche |  |  | 2 |  |
Vesting period (in years) |  |  | 4 years |  |
Compensation expense |  |  |  |  | 2,035,000
Equity instruments other than options, grants in period (in shares) | shares |  |  | 873 |  |
RSUs outstanding | Research and development |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Compensation expense |  |  |  |  | 1,332,000
RSUs outstanding | Sales and marketing |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Compensation expense |  |  |  |  | 210,000
RSUs outstanding | General and administrative |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Compensation expense |  |  |  |  | 493,000
Founder awards where performance has been met |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Compensation expense |  |  | $ 26,622,000 | $ 30,997,000 | $ 16,463,000
Founder awards where performance has been met | Founders |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Equity instruments other than options, grants in period (in shares) | shares |  | 1,800 |  |  |
Number of tranches | tranche |  |  | 10 |  |
Capitalized Software |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Estimated useful life (in years) |  |  | 3 years |  |
Accounts Receivable | Customer Concentration Risk | Apple |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk (in percent) |  |  | 65.20% | 56.20% |
Accounts Receivable | Customer Concentration Risk | Google |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk (in percent) |  |  | 20.70% | 27.50% |
Accounts Receivable | Customer Concentration Risk | Stripe |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk (in percent) |  |  | 10.70% |  |
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Apple |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk (in percent) |  |  | 58.50% | 54.20% | 50.50%
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Google |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk (in percent) |  |  | 25.60% | 28.10% | 29.00%
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Stripe |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk (in percent) |  |  | 12.10% |  | 10.10%

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