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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Energy Related Inventory    
Natural gas in storage $ 251.9 $ 227.7
Materials and supplies 247.3 234.2
Fossil fuel 74.2 87.9
Total 573.4 $ 549.8
LIFO Method Related Items    
LIFO liquidation debit $ 0.0