STOCK-BASED COMPENSATION - Additional Information (Details)	1 Months Ended	9 Months Ended
	Mar. 31, 2021	Sep. 30, 2021 USD ($) plan hour $ / shares shares	Sep. 30, 2020 USD ($) $ / shares
Share-based Payment Arrangement [Abstract]
Number of equity plans | plan		3
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)		4 years
Weighted average period for recognition of compensation costs (in years)		1 year 4 months 24 days
Expiration period of stock option plan (in years)		10 years
Options granted to employees (in shares)		0
Weighted average grant date fair value of options granted, per share (in USD per share) | $ / shares			$ 1.20
Unrecognized compensation cost | $		$ 5,800,000
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value of RSUs granted (in USD per share) | $ / shares		$ 4.82	$ 2.78
Restricted Stock Units (RSUs) | Employees and/or Board of Directors | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)		1 year
Restricted Stock Units (RSUs) | Employees and/or Board of Directors | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)		3 years
Incentive Stock Units (ISUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value of RSUs granted (in USD per share) | $ / shares		$ 4.82
Total grant date fair value of ISUs | $		$ 16,100,000	$ 16,700,000
Total fair value of ISUs vested | $		24,200,000	$ 2,700,000
Unrecognized stock based compensation cost | $		$ 17,200,000
Weighted average period for recognition of compensation costs (in years)		1 year 9 months 18 days
Shares expected to vest (in shares)		6,400,000
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Minimum hours worked per week | hour		20
Minimum months worked (in months)		5 months
Maximum contribution percentage amount of employee's base compensation (as a percent)		15.00%
Maximum employee contribution in to share purchase plan per offering | $		$ 30,000
Maximum number of shares allowed for purchase by each employee (in shares)		3,000
Purchase price as a percentage of fair market value per share (as a percent)		85.00%
Common stock reserved for issuance (in shares)		3,500,000
Number of common stock authorized for issuance (in shares)		3,500,000
Shares issued under ESPP (in shares)		300,000
Common stock remained available for issuance pursuant to the purchase plan (in shares)		3,200,000
2015 and 2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for grant (in shares)		5,700,000
2021 PSU Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of years of compound annual revenue growth rate used as metric to award performance share units (in years)	3 years