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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Energy Related Inventory    
Materials and supplies $ 218.3 $ 218.1
Natural gas in storage 68.3 224.9
Fossil fuel 66.9 85.6
Total 353.5 $ 528.6
LIFO Method Related Items    
LIFO liquidation credit $ 66.3