INCOME TAXES - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets
Net operating loss carryforwards	$ 121,839	$ 172,359
Research and development tax credit carryforwards	55,211	55,966
Other	1,100	1,412
Total deferred tax assets before valuation allowance	178,150	229,737
Valuation allowance	(63,423)	(63,620)
Total deferred tax assets	114,727	166,117
Deferred tax liabilities
Debt issuance discount and other	(10,596)	(11,946)
Unrealized Gain on Investment	(10,372)	0
Net deferred tax assets	$ 93,759	$ 154,171