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Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disaggregation of Revenue [Line Items]      
Customer revenues $ 4,991.1 [1] $ 4,730.2 $ 4,392.5
Other revenues 123.4 144.4 100.0
Total Operating Revenues 5,114.5 $ 4,874.6 $ 4,492.5
Gas Distribution Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 3,372.0    
Other revenues 34.4    
Total Operating Revenues 3,406.4    
Electric Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 1,618.4    
Other revenues 89.0    
Total Operating Revenues 1,707.4    
Corporate and Other      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 0.7    
Other revenues 0.0    
Total Operating Revenues 0.7    
Residential | Gas Distribution Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 2,250.0    
Residential | Electric Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 494.7    
Residential | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 0.0    
Commercial | Gas Distribution Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 751.9    
Commercial | Electric Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 492.7    
Commercial | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 0.0    
Industrial | Gas Distribution Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 228.0    
Industrial | Electric Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 613.6    
Industrial | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 0.0    
Off-system | Gas Distribution Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 92.4    
Off-system | Electric Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 0.0    
Off-system | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 0.0    
Miscellaneous | Gas Distribution Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 49.7    
Miscellaneous | Electric Operations      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 17.4    
Miscellaneous | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 0.7    
Residential      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 2,744.7    
Commercial      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 1,244.6    
Industrial      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 841.6    
Off-system      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] 92.4    
Miscellaneous      
Disaggregation of Revenue [Line Items]      
Customer revenues [1] $ 67.8    
[1] Customer revenue amounts exclude intersegment revenues. See Note 22, "Segments of Business," for discussion of intersegment revenues.