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REVENUE RECOGNITION AND RECEIVABLES - Accounts Receivable (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Accounts Receivable      
Gross accounts receivable $ 78,000 $ 67,100  
Allowance for credit loss 13,000 12,100  
Customer receivable, current 2,365 1,227  
Customer receivable - long term 21,056 9,614  
Retained earnings 516,897 516,901  
FirstNet agreement      
Accounts Receivable      
Customer receivable 23,400 10,800  
Customer receivable, current 2,400 1,200  
Customer receivable - long term $ 21,000 $ 9,600  
Cumulative Effect, Period of Adoption, Adjustment | ASU 2016-13      
Accounts Receivable      
Retained earnings     $ 0