CONVERTIBLE PREFERRED STOCK - Narrative (Details)	1 Months Ended						12 Months Ended
	Nov. 30, 2019 USD ($) $ / shares shares	Jul. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 shares	Mar. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) director $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			44,867,089				64,871,795	44,867,089
Convertible preferred stock, par value (in dollars per share) | $ / shares			$ 0.0001				$ 0.0001	$ 0.0001
Number of directors | director							2
Conversion terms, trigger stock price (in dollars per share) | $ / shares							$ 4.50
Conversion terms, trigger gross proceeds | $							$ 75,000,000
Preferred stock dividends | $							$ 0	$ 0
Series A
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			22,533,945				22,533,945	22,533,945
Number of shares sold				5,000,000	5,000,000	5,000,000
Stock price (in dollars per share) | $ / shares			$ 1.00	$ 1.00			$ 1.00	$ 1.00
Proceeds from stock issuance, net | $				$ 4,899,000		$ 4,924,000
Stock issuance costs | $				$ 101,000		$ 76,000
Number of shares issued upon conversion						2,501,503
Derivative liabilities | $						$ 500,000
Stock price, post stock split (in dollars per share) | $ / shares			$ 3.02				$ 3.02	$ 3.02
Number of shares issued upon conversion			1				1	1
Series B
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			22,333,144				22,917,726	22,333,144
Number of shares sold		456,252	11,166,572	11,294,902
Stock price (in dollars per share) | $ / shares		$ 3.81	$ 3.81			$ 1.00	$ 3.81	$ 3.81
Proceeds from stock issuance, net | $		$ 1,736,000	$ 42,414,000
Stock issuance costs | $			86,000
Derivative liabilities | $			$ 4,000,000					$ 4,000,000
Stock price, post stock split (in dollars per share) | $ / shares			$ 11.48				$ 11.48	$ 11.48
Number of shares issued upon conversion			1				1	1
Series C
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized							19,420,124
Number of shares sold	19,420,124
Stock price (in dollars per share) | $ / shares	$ 4.38						$ 4.38
Proceeds from stock issuance, net | $	$ 84,737,000
Stock issuance costs | $	$ 263,000
Stock price, post stock split (in dollars per share) | $ / shares							$ 13.20
Number of shares issued upon conversion							1
Developmental milestone | Series A
Temporary Equity [Line Items]
Stock price (in dollars per share) | $ / shares						$ 1.00
Proceeds from stock issuance, net | $						$ 5,000,000
Number of shares reserved for future issuance						5,000,000
Provision of occurrence before December 15, 2018 | Series A
Temporary Equity [Line Items]
Stock price (in dollars per share) | $ / shares				$ 1.00
Number of shares reserved for future issuance				5,000,000
Proceeds from stock issuance, gross | $				$ 5,000,000
Regulatory milestone | Series B
Temporary Equity [Line Items]
Stock price (in dollars per share) | $ / shares			$ 3.81					$ 3.81
Proceeds from stock issuance, net | $								$ 42,500,000
Number of shares reserved for future issuance			11,166,572					11,166,572
Series B tranche right provision | Series B
Temporary Equity [Line Items]
Stock price (in dollars per share) | $ / shares		$ 3.81
Number of shares reserved for future issuance		128,330