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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fixed Assets (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Fixed assets    
Accrued asset retirement obligations $ 3.8 $ 3.9
Minimum    
Fixed assets    
Useful life 3 years  
Maximum    
Fixed assets    
Useful life 39 years