CONVERTIBLE PREFERRED STOCK - Schedule of Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	64,871,795	44,867,089
Convertible preferred stock, shares issued	64,839,353	33,668,075
Convertible preferred stock, shares outstanding	64,839,353	33,668,075	12,501,503
Carrying value	$ 200,573	$ 60,770	$ 12,458
Convertible preferred stock, liquidation preference	$ 194,727	$ 65,002
Common stock issuable upon conversion (in shares)	21,499,770	11,163,845
Series A
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	22,533,945	22,533,945
Convertible preferred stock, shares issued	22,501,503	22,501,503
Convertible preferred stock, shares outstanding	22,501,503	22,501,503
Carrying value	$ 22,357	$ 22,357
Convertible preferred stock, liquidation preference	$ 22,502	$ 22,502
Common stock issuable upon conversion (in shares)	7,461,168	7,461,168
Series B
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	22,917,726	22,333,144
Convertible preferred stock, shares issued	22,917,726	11,166,572
Convertible preferred stock, shares outstanding	22,917,726	11,166,572
Carrying value	$ 93,479	$ 38,413
Convertible preferred stock, liquidation preference	$ 87,225	$ 42,500
Common stock issuable upon conversion (in shares)	7,599,178	3,702,677
Series C
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	19,420,124
Convertible preferred stock, shares issued	19,420,124
Convertible preferred stock, shares outstanding	19,420,124
Carrying value	$ 84,737
Convertible preferred stock, liquidation preference	$ 85,000
Common stock issuable upon conversion (in shares)	6,439,424