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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) shares in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2021 shares	Dec. 31, 2024 USD ($) reporting_unit tranche revenueSource shares	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Concentration Risk [Line Items]
Number of sources of revenue \| revenueSource		4
Number of days to take exam		21 days
Advertising costs		$ 62,373,000	$ 52,969,000	$ 48,111,000
Compensation expense		$ 110,477,000	95,221,000	73,820,000
Number of reporting units \| reporting_unit		1
Impairment		$ 0	0	0
Impairment of capitalized software		$ 238,000	0	0
Impairment, Long-Lived Asset, Held-for-Use, Statement of Income or Comprehensive Income [Extensible Enumeration]		Impairment of capitalized software
RSUs outstanding
Concentration Risk [Line Items]
Vesting period (in years)		4 years
Equity instruments other than options, grants in period (in shares) \| shares		719
Founder awards where performance has been met
Concentration Risk [Line Items]
Compensation expense		$ 19,799,000	$ 26,622,000	$ 30,997,000
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
Apple \| Accounts Receivable \| Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk (in percent)		64.70%	65.20%
Google \| Accounts Receivable \| Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk (in percent)		21.20%	20.70%
Stripe \| Accounts Receivable \| Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk (in percent)			10.70%
Capitalized Software
Concentration Risk [Line Items]
Estimated useful life (in years)		3 years