STOCKHOLDERS' EQUITY - Common stock (Details) - shares	Mar. 31, 2020	Dec. 31, 2019
Equity [Abstract]
Stock options and RSUs issued and outstanding (in shares)	9,009,511	6,809,257
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)	3,395,164	3,257,316
Total (in shares)	12,404,675	10,066,573