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CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFICIT - Convertible Preferred Stock (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) Vote $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2021 USD ($) shares	Dec. 31, 2018 shares
Temporary Equity [Line Items]
Authorized shares | shares	409,351,021	321,346,716	409,351,021
Outstanding shares | shares	175,437,783	191,139,933	175,528,225	186,466,181
Liquidation preference | $	$ 257,947	$ 258,041	$ 258,084
Conversion upon public offering with gross proceeds | $	$ 20,000
Conversion upon consent of percentage of holders	66.00%
No dividends declared	$ 0	$ 0
No dividends paid	$ 0	$ 0
Common stock voting rights per share | Vote	1
Preferred stock voting rights, percentage of votes	65.00%
Series D'
Temporary Equity [Line Items]
Authorized shares | shares	190,000,000	102,000,000	190,000,000
Outstanding shares | shares	105,386,149	78,932,752	105,386,149
Net carrying value | $	$ 129,339	$ 99,588
Liquidation preference | $	$ 161,516	$ 120,972	$ 161,515
Issue price	$ 1.5326
Dividend rate per share	0.1226
Liquidation preference per share	$ 1.5326	$ 1.5326
Series D
Temporary Equity [Line Items]
Authorized shares | shares	87,235,535	87,235,535	87,235,535
Outstanding shares | shares	33,392,701	35,586,652	33,483,143
Net carrying value | $	$ 46,587	$ 49,772
Liquidation preference | $	$ 51,178	$ 54,540	$ 51,316
Issue price	$ 1.5326
Dividend rate per share	0.1226
Liquidation preference per share	$ 1.5326	$ 1.5326
Series C
Temporary Equity [Line Items]
Authorized shares | shares	64,129,209	64,129,209
Outstanding shares | shares	23,298,388	41,305,384
Net carrying value | $	$ 35,293	$ 62,570
Liquidation preference | $	$ 35,514	$ 62,962
Issue price	$ 1.5243
Dividend rate per share	0.1219
Liquidation preference per share	$ 1.5243	$ 1.5243
Series B
Temporary Equity [Line Items]
Authorized shares | shares	36,969,407	36,969,407	36,969,407
Outstanding shares | shares	9,185,302	9,126,596	9,185,302
Net carrying value | $	$ 7,049	$ 7,071
Liquidation preference | $	$ 7,123	$ 7,078	$ 7,123
Issue price	$ 0.7755
Dividend rate per share	0.0620
Liquidation preference per share	$ 0.7755	$ 0.7755
Series A'
Temporary Equity [Line Items]
Authorized shares | shares	30,991,277	30,991,277
Outstanding shares | shares	4,158,503	26,171,809
Net carrying value | $	$ 1,702	$ 11,535
Liquidation preference | $	$ 1,865	$ 11,738
Issue price	$ 0.4485
Dividend rate per share	0.0359
Liquidation preference per share	$ 0.4485	$ 0.4485
Series A
Temporary Equity [Line Items]
Authorized shares | shares	21,288	21,288
Outstanding shares | shares	16,740	16,740
Net carrying value | $	$ 593	$ 593
Liquidation preference | $	$ 751	$ 751
Issue price	$ 0.4485
Dividend rate per share	3.59
Liquidation preference per share	$ 44.85	$ 44.85
Series 1
Temporary Equity [Line Items]
Authorized shares | shares	4,305	4,305
Outstanding shares | shares	2,961	2,961
Issue price	$ 1.00

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Amount that would be paid, determined under the conditions specified in the contract, if the holder of the share has the right to redeem the shares.

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The aggregate liquidation preference (or restrictions) of stock classified as temporary equity that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

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The per share liquidation preference (or restrictions) of stock classified as temporary equity that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

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