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Summary of Significant Accounting Policies Fuel Inventories (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2014
Other $ 54.6 $ 81.8  
Public Utilities, Inventory, Fuel [Member]      
Public Utilities, Inventory 84.3 $ 82.4  
Other Assets     $ 11.0
Other $ 2.8