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MAJOR CUSTOMERS - (Details) - Customer
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Revenue | Verizon Communications Inc.      
MAJOR CUSTOMERS      
Concentration risk, percentage 17.00% 17.00% 17.00%
Revenue | AT&T Inc.      
MAJOR CUSTOMERS      
Concentration risk, percentage 12.00%    
Accounts receivable      
MAJOR CUSTOMERS      
Concentration risk, percentage 22.00% 32.00%