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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Energy Related Inventory    
Natural gas in storage $ 353.2 $ 446.3
Materials and supplies 301.7 257.0
Fossil fuel 95.0 103.8
Total 749.9 $ 807.1
LIFO Method Related Items    
LIFO liquidation balance $ 0.0