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MAJOR CUSTOMERS - (Details) - Customer
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenue | Verizon Communications Inc.      
MAJOR CUSTOMERS      
Concentration risk, percentage 17.00% 17.00%  
Revenue | AT&T Inc.      
MAJOR CUSTOMERS      
Concentration risk, percentage     12.00%
Accounts receivable      
MAJOR CUSTOMERS      
Concentration risk, percentage 32.00% 31.00%