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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Inventory Disclosure [Abstract]    
Natural gas in storage $ 137.8 $ 223.1
Materials and supplies 216.3 206.5
Fossil fuel 155.9 158.0
Total $ 510.0 $ 587.6