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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 |

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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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