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Summary of Significant Accounting Policies Accounting Records (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Regulatory Assets, Current $ 48.7 $ 18.5
Regulatory Assets, Noncurrent 306.0 285.8
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year (1.7)  
Deferred Storm and Property Reserve Deficiency, Current 2.7  
Regulatory Liability, Current 29.0 36.6
Pension Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]    
Components of Benefit Obligation Regulatory Asset 160.5 185.3
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year 11.4  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]    
Components of Benefit Obligation Regulatory Asset 23.3 25.6
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year 2.8  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]    
Components of Benefit Obligation Regulatory Asset (16.6) $ (22.7)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year (6.1)  
Regulatory Asset [Member]    
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year $ 8.1