STOCKHOLDERS' (DEFICIT) EQUITY - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Class of Warrant or Right [Line Items]
Common stock, shares issued (in shares)	140,449,647	106,801,409
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	10,147,000	6,236,000
Total shares of common stock issued and reserved for issuance (in shares)	157,707,000	144,713,000
Common stock, shares outstanding (in shares)	140,449,647	106,801,409
Warrants
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	2,247,000	22,895,000
Shares available for grant under 2014 Stock Incentive Plan
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	4,863,000	8,753,000
Shares available for sale under 2014 Employee Stock Purchase Plan
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	0	28,000