STOCKHOLDERS' EQUITY (DEFICIT) - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Mar. 31, 2021	Dec. 31, 2020
Class of Warrant or Right [Line Items]
Common stock, shares issued (in shares)	171,978,799	140,449,647
Shares of common stock reserved for issuance for:
Total shares of common stock issued and reserved for issuance (in shares)	188,513,000	157,707,000
Warrants
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	1,394,000	2,247,000
Stock options
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	14,380,000	10,147,000
Shares available for grant under 2014 Stock Incentive Plan
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	760,000	4,863,000