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COMMITMENTS AND CONTINGENCIES - Purchase Commitments (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Mar. 09, 2018
Contractual Commitments    
Commitments to purchase coal   $ 2.0
Commitments for external coal purchases    
Contractual Commitments    
Commitments to purchase coal $ 0.0  
Commitments related to planned capital projects    
Contractual Commitments    
Contractual amount $ 28.6