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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Energy Related Inventory    
Natural gas in storage $ 365.3 $ 224.9
Materials and supplies 229.0 218.1
Fossil fuel 51.4 85.6
Total 645.7 $ 528.6
LIFO Method Related Items    
LIFO liquidation balance $ 0.0