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Regulatory Matters (Regulatory Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Regulated Operations [Line Items]    
Total Assets $ 2,460.2 $ 2,580.8
Regulatory Assets, Current 214.3 233.2
Regulatory Assets, Noncurrent $ 2,245.9 2,347.6
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided 69 years  
Unrecognized Pension Benefit And Other Postretirement Benefit Costs    
Regulated Operations [Line Items]    
Total Assets $ 561.6 607.5
Other Postretirement Costs    
Regulated Operations [Line Items]    
Total Assets 59.7 72.2
Environmental Costs    
Regulated Operations [Line Items]    
Total Assets 40.6 41.4
Regulatory Effects Of Accounting For Income Taxes    
Regulated Operations [Line Items]    
Total Assets 163.5 158.0
Underrecovered Gas And Fuel Costs    
Regulated Operations [Line Items]    
Total Assets 12.7 85.5
Depreciation    
Regulated Operations [Line Items]    
Total Assets 201.9 191.3
Post-In Service Carrying Charges    
Regulated Operations [Line Items]    
Total Assets 269.9 251.5
Safety Activity Costs    
Regulated Operations [Line Items]    
Total Assets 206.6 200.7
DSM Program    
Regulated Operations [Line Items]    
Total Assets 25.0 37.5
Retired coal generating stations    
Regulated Operations [Line Items]    
Total Assets 682.0 744.0
Deferred Derivative Gain (Loss)    
Regulated Operations [Line Items]    
Total Assets 24.4 10.0
Deferred Property Taxes    
Regulated Operations [Line Items]    
Total Assets 72.3 68.5
Renewable Energy Program    
Regulated Operations [Line Items]    
Total Assets 60.8 37.7
Other Assets    
Regulated Operations [Line Items]    
Total Assets $ 79.2 $ 75.0