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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Mar. 31, 2022
Dec. 31, 2021
Energy Related Inventory    
Materials and supplies $ 232.2 $ 225.3
Fossil fuel 81.3 84.5
Natural gas in storage 55.3 326.0
Total 368.8 $ 635.8
LIFO Method Related Items    
LIFO liquidation balance $ 130.7