RELATED PARTY TRANSACTIONS - Narrative (Details)				1 Months Ended						3 Months Ended			9 Months Ended
	Jul. 22, 2020 shares	Jun. 03, 2020 shares	Jun. 02, 2020 $ / shares	Jun. 30, 2020 USD ($)	May 31, 2020 shares	Mar. 31, 2020 USD ($) $ / shares shares	Oct. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares		Sep. 30, 2020 USD ($) $ / shares shares		Sep. 30, 2019 USD ($) shares		Dec. 31, 2019 USD ($) $ / shares
Related Party Transaction [Line Items]
Options granted (in shares) | shares													1,582,496
Weighted average grant date fair value (in dollars per share) | $ / shares													$ 6.92
Stock-based compensation expense										$ 52,196,000	$ 1,185,000		$ 91,736,000		$ 3,772,000
Series D redeemable convertible preferred stock in exchange for licensed technology (in shares) | shares										0			5,132,289
Issuance of Series D redeemable convertible preferred stock, net of issuance costs (in shares) | shares	22,877,806
Less: Premium on repurchase of redeemable convertible preferred stock										$ 0	$ 0		$ 13,407,000		$ 0
M&M Residual, LLC
Related Party Transaction [Line Items]
Stock repurchased during the period (in shares) | shares		7,000,000
Shares repurchased, price per share (in usd per share) | $ / shares			$ 10.00
Common Stock
Related Party Transaction [Line Items]
Issuance of Series D redeemable convertible preferred stock, net of issuance costs (in shares) | shares											6,671,998	[1]	6,581,340	[2]	6,671,998	[3]
Issuance of Series D redeemable convertible preferred stock for in-kind contribution (in shares) | shares											5,132,291	[1]	9,443,353	[2]	5,132,291	[3]
Series D Preferred Stock
Related Party Transaction [Line Items]
Stock issued during period (in shares) | shares								25,661,448
Sale of stock, consideration received on transaction, license value								$ 50,000,000.0
Sale of stock, consideration received on transaction, value of in-kind services								100,000,000.0		0			$ 50,000,000.0
Sale of stock, consideration received on transaction								$ 100,000,000.0			$ 50,000,000.0				$ 50,000,000.0
Series D redeemable convertible preferred stock in exchange for licensed technology (in shares) | shares											5,132,291				5,132,291
Issuance of Series D redeemable convertible preferred stock, net of issuance costs (in shares) | shares											5,132,291				5,132,291
Convertible preferred stock per share (in usd per share) | $ / shares																	$ 9.74
Chief Executive Officer
Related Party Transaction [Line Items]
Accounts receivable, related parties, current										0			$ 0				$ 50,000.00
Chief Executive Officer | Reissued Performance Stock
Related Party Transaction [Line Items]
Options granted (in shares) | shares					180,153				6,005,139
Weighted average grant date fair value (in dollars per share) | $ / shares													$ 1.20
Stock-based compensation expense				$ 7,200,000
Chief Executive Officer | Aircraft Charter
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party										1,070,000.00	$ 60,000.00		$ 1,310,000		$ 100,000
Accounts payable, related parties										970,000			970,000				30,000.00
Affiliated Entity
Related Party Transaction [Line Items]
Accounts payable, related parties										400,000			400,000				600,000
Stock repurchased during the period (in shares) | shares						2,850,930
Shares repurchased, price per share (in usd per share) | $ / shares						$ 8.77
Accrued expenses with related parties										$ 7,400,000			$ 7,400,000				500,000
Stock repurchased during the period						$ 25,000,000.0
Affiliated Entity | Common Stock
Related Party Transaction [Line Items]
Issuance of Series D redeemable convertible preferred stock for in-kind contribution (in shares) | shares										0			9,443,353
Issuance of Series D redeemable convertible preferred stock for in-kind contribution, gross										$ 0			$ 92,000,000.0
Affiliated Entity | Related Party Research And Development Expense
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party										4,100,000	$ 2,300,000		10,600,000		$ 13,000,000.0
Affiliated Entity | Prepaid In-Kind Services
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party										11,400,000			28,600,000
Due from Related Parties										$ 63,400,000			$ 63,400,000				$ 0
Affiliated Entity | Series D Preferred Stock
Related Party Transaction [Line Items]
Percentage of financing redeemed																	0.05
Affiliated Entity | Series B Preferred Stock
Related Party Transaction [Line Items]
Temporary equity repurchased (in shares) | shares								3,575,750
Temporary equity repurchased, price per share (in usd per share) | $ / shares								$ 8.77
Share price as percentage of financing																	0.90
Temporary equity repurchased and retired during period, value							$ 31,400,000

[1] Issuance of Series D redeemable convertible preferred stock has been retroactively restated to give effect to the recapitalization transaction.

[2] Issuance of Series D redeemable convertible preferred stock has been retroactively restated to give effect to the recapitalization transaction.

[3] Issuance of Series D redeemable convertible preferred stock has been retroactively restated to give effect to the recapitalization transaction.