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Summary of Significant Accounting Policies Accounting Records (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Regulatory Assets, Current $ 121.8 $ 46.5
Regulatory Assets, Noncurrent 404.8 402.2
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year (3.9)  
Deferred Storm and Property Reserve Deficiency, Current 2.7  
Smart Grid Project [Member]    
Smart Grid Costs to be Recovered 6.9  
Benefit Obligations [Member]    
Regulatory Assets, Noncurrent 232.6 242.2
Regulatory Asset [Member]    
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year 14.7  
Pension Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]    
Components of Benefit Obligation Regulatory Asset 199.9 214.1
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year 12.4  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]    
Components of Benefit Obligation Regulatory Asset 32.7 34.2
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year 2.3  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]    
Components of Benefit Obligation Regulatory Asset $ 0.0 $ (6.1)