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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Energy Related Inventory    
Materials and supplies $ 240.2 $ 234.2
Natural gas in storage 142.5 227.7
Fossil fuel 85.0 87.9
Total 467.7 $ 549.8
LIFO Method Related Items    
LIFO liquidation debit $ 12.6