STOCKHOLDERS' EQUITY - Common Stock (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Stock options and RSUs issued and outstanding (in shares)	6,809,257	4,599,307
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)	3,257,316	4,534,784
Total (in shares)	10,066,573	9,134,091