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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Energy Related Inventory    
Natural gas in storage $ 266.4 $ 232.9
Materials and supplies 236.7 226.6
Fossil fuel 90.2 88.7
Total 593.3 $ 548.2
LIFO Method Related Items    
Lifo liquidation balance $ 0.0