INCOME TAXES - Summary of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Net operating loss carryforwards	$ 295,985	$ 223,685
Research and development credit	35,665	27,558
Noncash compensation	32,356	22,381
Disallowed interest	2,434	1,855
Other	1,006	1,224
Deferred tax asset, excluding valuation allowance	367,446	276,703
Less valuation allowance	367,446	276,703
Net deferred tax assets	$ 0	$ 0