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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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