STOCKHOLDERS' EQUITY (DEFICIT) - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Sep. 30, 2021	Dec. 31, 2020	Feb. 28, 2014
Class of Warrant or Right [Line Items]
Common stock, shares issued (in shares)	199,463,645	140,449,647
Shares of common stock reserved for issuance for:
Total shares of common stock issued and reserved for issuance (in shares)	229,457,000	157,707,000
2014 ESPP
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)			200,000
Warrants
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	199,000	2,247,000
Stock options
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	15,511,000	10,147,000
Stock options | 2014 Stock Incentive Plan
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	12,000,000
Share-based payment arrangement | 2014 Stock Incentive Plan
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	11,983,000	4,863,000
Share-based payment arrangement | 2014 ESPP
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	2,300,000	0