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Income Tax Expense (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Current Tax Expense (Benefit) [Abstract]          
Federal $ (0.2) [1] $ 0 [1] $ 0 [1] $ 0 [1]  
State 0 [1] 0 [1] 0 [1] 0 [1]  
Total Current Tax Expense (Benefit) (0.2) 0 0 0  
Deferred Tax Expense (Benefit) [Abstract]          
Federal 3.6 4.8 8.2 13.2  
State 0.9 [2] 0.2 [2] 3.8 [2] 0.3 [2]  
Investment Tax Credit Amortization (0.2) (0.2) (0.4) (0.4)  
Total Deferred Tax Expense 4.3 4.8 11.6 13.1  
Total Income Tax Expense 4.1 4.8 11.6 13.1  
Effective Income Tax Rate     20.00% 25.20%  
Federal Statutory Income Tax Rate     41.00%    
Uncertain Tax Positions [Abstract]          
Gross Unrecognized Tax Benefits 2.6   2.6   2.7
Unrecognized Tax Benefits that Would Favorably Impact Effective Tax Rate $ 0.7   $ 0.7    
[1] We incurred net operating losses (NOLs) due to the bonus depreciation provisions of the American Taxpayer Relief Act of 2012 and the Tax Relief Unemployment Insurance Reauthorization and Job Creation Act of 2010 for the quarter and six months ended June 30, 2013 and 2012. The 2013 and 2012 federal and state NOLs will be carried forward to offset future taxable income.
[2] The quarter and six months ended June 30, 2012, reflected increased state tax benefit from state renewable tax credits compared to the quarter and six months ended June 30, 2013.