FAIR VALUE MEASUREMENTS - Financial assets and liabilities that are measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Liabilities
Warrant liability	$ 18,667	$ 0
Fair Value, Recurring
Assets
Total assets	471,999	60,529
Liabilities
Warrant liability	18,667
Derivative liabilities		13,390
Total liabilities	18,667	13,390
Fair Value, Recurring | Cash
Assets
Cash and cash equivalents, fair value disclosure	861	1,244
Fair Value, Recurring | Money market funds
Assets
Cash and cash equivalents, fair value disclosure	471,138	59,285
Level 1 | Fair Value, Recurring
Assets
Total assets	471,999	60,529
Liabilities
Warrant liability	18,667
Derivative liabilities		0
Total liabilities	18,667	0
Level 1 | Fair Value, Recurring | Cash
Assets
Cash and cash equivalents, fair value disclosure	861	1,244
Level 1 | Fair Value, Recurring | Money market funds
Assets
Cash and cash equivalents, fair value disclosure	471,138	59,285
Level 2 | Fair Value, Recurring
Assets
Total assets	0	0
Liabilities
Warrant liability	0
Derivative liabilities		0
Total liabilities	0	0
Level 2 | Fair Value, Recurring | Cash
Assets
Cash and cash equivalents, fair value disclosure	0	0
Level 2 | Fair Value, Recurring | Money market funds
Assets
Cash and cash equivalents, fair value disclosure	0	0
Level 3 | Fair Value, Recurring
Assets
Total assets	0	0
Liabilities
Warrant liability	0
Derivative liabilities		13,390
Total liabilities	0	13,390
Level 3 | Fair Value, Recurring | Cash
Assets
Cash and cash equivalents, fair value disclosure	0	0
Level 3 | Fair Value, Recurring | Money market funds
Assets
Cash and cash equivalents, fair value disclosure	$ 0	$ 0