INCOME TAXES - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets
Net operating loss carryforwards	$ 172,359	$ 215,497
Research and development tax credit carryforwards	55,966	56,231
Other	1,412	1,647
Total deferred tax assets before valuation allowance	229,737	273,375
Valuation allowance	(63,620)	(64,199)
Total deferred tax assets	166,117	209,176
Deferred tax liabilities
Debt issuance discount and other	(11,946)	(13,122)
Net deferred tax assets	$ 154,171	$ 196,054