STOCKHOLDERS' EQUITY - Preferred stock issued (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
	May 16, 2019	Feb. 28, 2019	Dec. 31, 2019	Mar. 31, 2020	Dec. 31, 2018
Preferred Stock
Convertible preferred stock, outstanding (in shares)	0		0	0
IPO
Preferred Stock
Issue price (in dollars per share)	$ 10.00
Conversion of convertible preferred stock into common stock upon closing of the IPO (in shares)	7,538,671
Number of shares issued for each share of convertible preferred stock converted	1
Series B Convertible Preferred Stock
Preferred Stock
Issuance of convertible preferred stock (in shares)		442,930	442,925
Issue price (in dollars per share)		$ 7.49
Gross proceeds from issuance of convertible preferred stock		$ 3.3
Issuance costs		$ 0.3
Convertible preferred stock, outstanding (in shares)					4,001,848