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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Non-Current Regulatory Liabilities $ 1,292.7 $ 1,283.4
Interim Rate Revenue Reserved 6.5  
Income taxes recoverable from customers, net [Member]    
Non-Current Regulatory Liabilities $ 951.3 $ 955.5