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Income Tax Expense (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Current Income Tax Expense [Abstract]          
Federal [1] $ 0.0 $ 0.0 $ 0.0 $ 0.0  
State [1] 0.1 0.0 0.3 0.2  
Total Current Income Tax Expense 0.1 0.0 0.3 0.2  
Deferred Income Tax Expense [Abstract]          
Federal 9.2 0.4 20.3 7.1  
State 5.0 1.4 14.5 8.9  
Investment Tax Credit Amortization (0.1) (0.1) (0.5) (0.5)  
Total Deferred Income Tax Expense 14.1 1.7 34.3 15.5  
Total Income Tax Expense 14.2 1.7 34.6 15.7  
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]          
Income Before Non-Controlling Interest and Income Taxes $ 59.1 $ 42.0 $ 165.4 $ 127.2  
Statutory Federal Income Tax Rate 35.00% 35.00% 35.00% 35.00%  
Income Taxes Computed at 35 percent Statutory Federal Rate $ 20.7 $ 14.7 $ 57.9 $ 44.5  
Increase (Decrease) in Tax Due to: [Abstract]          
State Income Taxes – Net of Federal Income Tax Benefit 3.3 0.9 9.6 5.9  
Production Tax Credits (10.4) (14.0) (33.4) (34.5)  
Other 0.6 0.1 0.5 (0.2)  
Total Income Tax Expense 14.2 $ 1.7 $ 34.6 $ 15.7  
Effective Tax Rate     20.90% 12.30%  
Uncertain Tax Positions [Abstract]          
Gross Unrecognized Tax Benefits 1.9   $ 1.9   $ 2.0
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate $ 0.7   $ 0.7    
[1] For the quarter and nine months ended September 30, 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.