INCOME TAXES - Additional Tax Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Accrued interest and penalties		$ 0	$ 0
Federal corporate tax rate (as a percent)		35.00%
Net decrease related to federal deferred tax assets and deferred tax liabilities		$ 124,000
Change in valuation allowance		124,000
Gross unrecognized tax benefits
Unrecognized tax benefits, beginning of year	$ 15,488	15,488	15,488	$ 15,459
Gross increase in tax portions		0	0	29
Unrecognized tax benefits, end of year		15,488	$ 15,488	$ 15,488
Forecast
Income Taxes
Federal corporate tax rate (as a percent)	21.00%
Federal
Income Taxes
Net operating loss carryforwards		1,000,000
Federal | Research and development
Income Taxes
Tax credit carryforward amount		45,200
State
Income Taxes
Net operating loss carryforwards		654,200
State | Research and development
Income Taxes
Tax credit carryforward amount		$ 32,300