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STOCKHOLDERS' EQUITY - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares | Jun. 30, 2022 | Dec. 31, 2021
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Class of Warrant or Right [Line Items] |  |
Shares of common stock issued (in shares) | 199,463,645 | 199,463,645
Shares of common stock reserved for issuance for: |  |
Total shares of common stock issued and reserved for issuance (in shares) | 230,263,000 | 229,640,000
Warrants |  |
Shares of common stock reserved for issuance for: |  |
Shares of common stock reserved for issuance (in shares) | 199,000 | 199,000
Stock options |  |
Shares of common stock reserved for issuance for: |  |
Shares of common stock reserved for issuance (in shares) | 17,161,000 | 15,703,000
Restricted stock units |  |
Shares of common stock reserved for issuance for: |  |
Shares of common stock reserved for issuance (in shares) | 8,063,000 | 3,041,000
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) | 3,076,000 | 8,933,000
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) | 2,300,000 | 2,300,000

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