INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Minimum tax and research and development tax credit carryforwards	$ 500
Tax reform, decrease to valuation allowance	700
Deferred tax asset, Valuation allowance	142,749	$ 132,712	$ 164,865	$ 148,310
Unrecognized tax positions	300	162	$ 187
Interest and penalties accrued	200	$ 100
U.S. Tax Authority
Income Tax [Line Items]
Net operating loss carryforwards	367,100
Operating loss carryforwards deduction of stock-based compensation	27,700
Deferred tax asset, Valuation allowance	137,100
Foreign Tax Authority
Income Tax [Line Items]
Net operating loss carryforwards	36,700
Deferred tax asset, Valuation allowance	5,600
Tax Years 2018 to 2037 | U.S. Tax Authority
Income Tax [Line Items]
Net operating loss carryforwards	331,700
No Expiration Date | U.S. Tax Authority
Income Tax [Line Items]
Net operating loss carryforwards	$ 35,400