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Revenue Recognition (Revenue Recognition Under Previous Guidance) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenue Recognition Under Previous Guidance [Line Items]      
Total Operating Revenues $ 5,114.5 $ 4,874.6 $ 4,492.5
Previous Accounting Guidance | Gas Distribution [Member]      
Revenue Recognition Under Previous Guidance [Line Items]      
Total Operating Revenues 2,348.4 2,063.2 1,850.9
Previous Accounting Guidance | Gas Transportation [Member]      
Revenue Recognition Under Previous Guidance [Line Items]      
Total Operating Revenues 1,055.2 1,021.5 964.6
Previous Accounting Guidance | Electric [Member]      
Revenue Recognition Under Previous Guidance [Line Items]      
Total Operating Revenues 1,707.4 1,785.5 1,660.8
Previous Accounting Guidance | Other [Member]      
Revenue Recognition Under Previous Guidance [Line Items]      
Total Operating Revenues $ 3.5 $ 4.4 $ 16.2