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DEBT - Assumed Senior Secured Credit Agreement (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Dec. 21, 2017
Oct. 27, 2017
Debt Instrument [Line Items]        
Debt average interest rate 5.96% 4.51%    
Revolving Credit Facility | Line of Credit        
Debt Instrument [Line Items]        
Line of credit potential increases     $ 100,000,000  
Silicon Valley Bank | Revolving Credit Facility | Line of Credit        
Debt Instrument [Line Items]        
Current borrowing capacity for line of credit $ 50,000,000      
Potential further increases 75,000,000      
Line of credit potential increases $ 125,000,000      
GENBAND        
Debt Instrument [Line Items]        
Debt average interest rate       4.67%
Line of Credit | GENBAND        
Debt Instrument [Line Items]        
Debt assumed as part of GENBAND merger       $ 17,900,000
Letter of Credit | GENBAND        
Debt Instrument [Line Items]        
Debt assumed as part of GENBAND merger       $ 2,900,000