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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Acquired Intangibles (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Telecommunications Licenses    
Intangible assets    
Impairment $ 0 $ 0
Customer relationships | Minimum    
Intangible assets    
Estimated life 7 years  
Customer relationships | Maximum    
Intangible assets    
Estimated life 13 years