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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Energy Related Inventory    
Materials and supplies $ 230.7 $ 234.2
Fossil fuel 92.5 87.9
Natural gas in storage 67.9 227.7
Total 391.1 $ 549.8
LIFO Method Related Items    
LIFO liquidation debit $ 11.3