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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Energy Related Inventory    
Materials and supplies $ 230.7 $ 226.6
Fossil fuel 60.4 88.7
Natural gas in storage 39.0 232.9
Total 330.1 $ 548.2
LIFO Method Related Items    
LIFO liquidation credit $ 40.2