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INCOME TAXES - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jun. 24, 2018
Jan. 01, 2018
Operating Loss Carryforwards [Line Items]          
Deferred tax assets, stock-based compensation $ 1,536 $ 727      
Tax credit carryforward $ 28,700        
Significant change in shareholder ownership, period of change 3 years        
Significant change in shareholder ownership, benchmark percentage 50.00%        
Decrease in deferred tax assets $ (9,152) (8,434)      
BEAT tax expense 400        
Income tax benefit (3,400) 18,440 $ (2,516)    
Deferred tax assets, valuation allowance 101,484 89,600      
Decrease in valuation allowance   200      
Potential penalties and interest 100 200 100    
Interest and penalties accrued in deferred tax liability 600 600 $ 200    
Internal Revenue Service (IRS)          
Operating Loss Carryforwards [Line Items]          
Operating loss carryforwards 289,100        
Domestic Tax Authority          
Operating Loss Carryforwards [Line Items]          
Deferred tax assets, valuation allowance 82,400 73,100      
Edgewater          
Operating Loss Carryforwards [Line Items]          
Net operating loss carryforwards acquired       $ 34,000  
Tax credit carryforwards acquired       $ 6,000  
Income tax benefit 700        
GENBAND          
Operating Loss Carryforwards [Line Items]          
Income tax benefit   16,400      
Deferred tax assets, valuation allowance 9,600        
IRELAND | GENBAND          
Operating Loss Carryforwards [Line Items]          
Deferred tax assets, valuation allowance $ 9,500 $ 6,200      
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09          
Operating Loss Carryforwards [Line Items]          
Decrease in deferred tax assets         $ 2,200