INCOME TAXES - Additional Tax Disclosures (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
Accrued interest and penalties	$ 0				$ 0				$ 0	$ 0
Uncertain Tax Positions
Unrecognized tax benefits	15,342,000				15,413,000				15,342,000	15,413,000	$ 15,488,000	$ 15,488,000
Gross decrease in tax portions for 2018										(75,000)
Gross decrease in tax portions for 2019									(71,000)
Unrecognized tax benefits would affect our effective income tax rate if recognized	8,000,000.0								8,000,000.0
Deferred
Federal									41,567,000	(190,195,000)	0
State									309,000	(5,859,000)	0
Total Income tax benefit (expense), net	(12,403,000)	$ (10,558,000)	$ (10,433,000)	$ (8,508,000)	$ 196,073,000	$ 0	$ 0	$ 0	(41,902,000)	$ 196,073,000	$ (4,000)
Federal
Income Taxes
Net operating loss carryforwards	600,000,000								600,000,000
Tax credit carryforward amount	44,400,000								44,400,000
State
Income Taxes
Net operating loss carryforwards	649,300,000								649,300,000
State | Research and development
Income Taxes
Tax credit carryforward amount	$ 32,300,000								$ 32,300,000