INCOME TAXES - Deferred Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Deferred tax assets
Net operating loss carryforwards	$ 257,000	$ 215,497
Research and development tax credit carryforwards	57,000	56,231
Other	3,000	1,647
Total deferred tax assets before valuation allowance	317,000	273,375
Valuation allowance	(303,000)	(64,199)
Total deferred tax assets	14,000	209,176
Deferred tax liabilities
Debt issuance discount and other	(14,000)	(13,122)
Net deferred tax assets	0	$ 196,054
Changes in valuation allowance
Increase (decrease) in valuation allowance	$ (181,200)