STOCKHOLDERS' EQUITY - PRICING MODEL ASSUMPTIONS (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Compensation cost related to unvested stock awards not yet recognized
Compensation cost not yet recognized, net of estimated forfeitures (in dollars)	$ 12,341,000
Period for recognition of compensation cost on weighted average basis	28 months
Stock Options
Assumptions used to estimate fair value of awards
Dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected volatility range, minimum (as a percent)	77.50%	78.10%	79.20%
Expected volatility range, maximum (as a percent)	78.30%	79.30%	98.00%
Risk-free interest rate range, minimum (as a percent)	0.51%	0.31%	1.50%
Risk-free interest rate range, maximum (as a percent)	1.30%	1.62%	2.56%
Expected term range	5 years 6 months	5 years 3 months
Additional disclosures
Weighted average estimated fair value of employee stock options granted (in dollars per share)	$ 1.17	$ 0.88	$ 0.94
Stock Options | Minimum
Assumptions used to estimate fair value of awards
Expected term range			5 years 3 months
Stock Options | Maximum
Assumptions used to estimate fair value of awards
Expected term range			6 years 5 months 8 days
Employee Stock Purchase Plan
Assumptions used to estimate fair value of awards
Dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected volatility range, minimum (as a percent)	50.70%	47.80%	64.60%
Expected volatility range, maximum (as a percent)	70.70%	81.80%	165.30%
Risk-free interest rate range, minimum (as a percent)	0.09%	0.16%	1.94%
Risk-free interest rate range, maximum (as a percent)	0.16%	1.57%	2.63%
Additional disclosures
Weighted average estimated fair value of purchase rights (in dollars per share)	$ 0.56	$ 0.62	$ 0.66
Employee Stock Purchase Plan | Minimum
Assumptions used to estimate fair value of awards
Expected term range	6 months	6 months	6 months
Employee Stock Purchase Plan | Maximum
Assumptions used to estimate fair value of awards
Expected term range	12 months	12 months	12 months