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REVENUES - Disaggregation of Revenues (Details)
$ in Millions
1 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
customer
Dec. 31, 2018
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Disaggregation of Revenues        
Provision For revenue sharing   $ 10.0    
Contract balances     $ 36.0 $ 37.0
Bison        
Disaggregation of Revenues        
Non-refundable receipt from contract termination   $ 97.0    
Number of customers that terminated transportation agreement | customer 2      
Bison | Tenaska        
Disaggregation of Revenues        
Non-refundable receipt from contract termination $ 95.4      
Bison | Another customer        
Disaggregation of Revenues        
Non-refundable receipt from contract termination $ 2.0