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STOCKHOLDERS' EQUITY DEFICIT - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Dec. 31, 2021	Dec. 31, 2020
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)	229,640,000	157,707,000
Warrants
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	199,000	2,247,000
Stock options
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	15,703,000
RSUs
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance (in shares)	3,041,000	0
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)	8,933,000	4,863,000
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	0

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Common Stock Shares Issued And Reserved For Future

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Shares Of Common Stock Reserved For Issuance [Abstract]

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Aggregate number of common shares reserved for future issuance.

+ 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)
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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.

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-Name Accounting Standards Codification
-Topic 210
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-Section S99
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