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GOODWILL AND INTANGIBLE ASSETS - Change In Carrying Amount Of Telecommunications Licenses (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Changes in the carrying amount of the company's telecommunications licenses, by operating segment        
Balance at the beginning of the period   $ 95,952    
Balance at the end of the period   93,686 $ 95,952  
Telecommunications Licenses        
Telecommunications licenses        
Discounted rate used for cash flows (as a percent) 14.50%      
Impairment of intangible assets $ 0 0 0 $ 0
Changes in the carrying amount of the company's telecommunications licenses, by operating segment        
Balance at the beginning of the period   95,952 48,291  
Acquired licenses   485 47,692  
Dispositions   (2,750) (31)  
Balance at the end of the period   93,687 95,952 48,291
Telecommunications Licenses | U.S. Telecom        
Changes in the carrying amount of the company's telecommunications licenses, by operating segment        
Balance at the beginning of the period   72,605 24,944  
Acquired licenses   485 47,692  
Dispositions   (2,750) (31)  
Balance at the end of the period   70,340 72,605 24,944
Telecommunications Licenses | International Telecom        
Changes in the carrying amount of the company's telecommunications licenses, by operating segment        
Balance at the beginning of the period   23,347 23,347  
Balance at the end of the period   $ 23,347 $ 23,347 $ 23,347