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Income Tax Expense (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Current Tax Expense [Abstract]      
Federal [1] $ 0.0 $ 0.0  
State [1] 0.1 0.1  
Total Current Tax Expense 0.1 0.1  
Deferred Tax Expense [Abstract]      
Federal 7.3 4.6  
State 5.9 4.8  
Investment Tax Credit Amortization (0.2) (0.2)  
Total Deferred Tax Expense 13.0 9.2  
Total Income Tax Expense 13.1 9.3  
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]      
Income Before Non-Controlling Interest and Income Taxes $ 62.1 $ 55.7  
Statutory Federal Income Tax Rate 35.00% 35.00%  
Income Taxes Computed at 35 percent Statutory Federal Rate $ 21.7 $ 19.5  
Increase (Decrease) in Tax [Abstract]      
State Income Taxes – Net of Federal Income Tax Benefit 3.9 3.2  
Production Tax Credits (13.0) (13.9)  
Regulatory Differences for Utility Plant 0.1 (0.1)  
Other 0.4 0.6  
Total Income Tax Expense $ 13.1 $ 9.3  
Effective Tax Rate 21.10% 16.70%  
Uncertain Tax Positions [Abstract]      
Gross Unrecognized Tax Benefits $ 2.0   $ 2.0
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate $ 0.7    
[1] For the three months ended March 31, 2017, and 2016, the federal and state current tax expense was minimal due to NOLs which resulted from the bonus depreciation provisions of the Protecting Americans from Tax Hikes Act of 2015, the Tax Increase Prevention Act of 2014 and the American Taxpayer Relief Act of 2012.