INCOME TAXES - Additional Tax Disclosures (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Accrued interest and penalties	$ 0	$ 0
Federal corporate tax rate (as a percent)	21.00%	35.00%
Uncertain Tax Positions
Unrecognized tax benefits, beginning of year	$ 15,488,000
Gross decrease in tax portions	(75,000.000)
Unrecognized tax benefits, end of year	15,413,000	$ 15,488,000
Unrecognized tax benefits would affect our effective income tax rate if recognized	8,000,000
Federal
Income Taxes
Net operating loss carryforwards	800,000,000
Federal | Research and development
Income Taxes
Tax credit carryforward amount	44,800,000
State
Income Taxes
Net operating loss carryforwards	653,000,000
State | Research and development
Income Taxes
Tax credit carryforward amount	$ 32,300,000