STOCKHOLDERS' EQUITY - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Common stock, shares issued (in shares)	101,267,578	77,456,180
Shares of common stock reserved for issuance for:
Total shares of common stock issued and reserved for issuance (in shares)	142,652,000	92,695,000
Warrants to purchase common stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	26,307,000	9,258,000
Stock options
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	6,450,000	3,942,000
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)	8,599,000	2,001,000
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)	28,000	38,000