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Regulatory Matters (Regulatory Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Regulatory Assets [Line Items]    
Total Assets $ 2,237.5 $ 1,801.2
Unrecognized Pension Benefit And Other Postretirement Benefit Costs    
Regulatory Assets [Line Items]    
Total Assets 798.3 733.5
Other Postretirement Costs    
Regulatory Assets [Line Items]    
Total Assets 74.1 70.7
Environmental Costs    
Regulatory Assets [Line Items]    
Total Assets 61.5 63.4
Regulatory Effects Of Accounting For Income Taxes    
Regulatory Assets [Line Items]    
Total Assets 233.1 238.8
Underrecovered Gas And Fuel Costs    
Regulatory Assets [Line Items]    
Total Assets 34.7 25.5
Depreciation    
Regulatory Assets [Line Items]    
Total Assets 209.6 181.0
Post-In Service Carrying Charges    
Regulatory Assets [Line Items]    
Total Assets 206.6 173.3
Safety Activity Costs    
Regulatory Assets [Line Items]    
Total Assets 91.7 66.5
DSM Program    
Regulatory Assets [Line Items]    
Total Assets 45.5 40.0
Bailly Generating Station [Member]    
Regulatory Assets [Line Items]    
Total Assets 244.3 0.0
Other Assets    
Regulatory Assets [Line Items]    
Total Assets $ 238.1 $ 208.5