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Selected Balance Sheet Data Inventory (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Public Utilities, Inventory $ 34.3 $ 33.9
Supplies [Member]    
Public Utilities, Inventory 25.8 25.7
Public Utilities, Inventory, Fuel [Member]    
Public Utilities, Inventory $ 8.5 $ 8.2