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Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Asset $ 22.5 $ 17.5
Non-Current Regulatory Asset 340.3 345.9
Total Regulatory Assets 362.8 363.4
Non-Current Regulatory Liability 60.1 43.5
Income Taxes [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liability 19.5 21.9
Plant Removal Obligations [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liability 18.1 15.0
Wholesale and Retail Contra AFUDC [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liability 15.5 1.5
Other [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liability 7.0 5.1
Deferred Fuel [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Asset 22.5 [1] 17.5 [1]
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Asset 260.7 292.8
Income Taxes [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Asset 36.0 28.6
Asset Retirement Obligation [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Asset 12.1 9.8
Cost Recovery Riders [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Asset 18.5 [2] 0.7 [2]
PPACA Income Tax Deferral [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Asset 5.0 5.0
Conservation Improvement Program [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Asset 4.3 4.6
Other [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Asset $ 3.7 $ 4.4
[1] Current regulatory assets are included in prepayments and other on our Consolidated Balance Sheet.
[2] The increase in cost recovery rider regulatory assets in 2012 is primarily due to revenues related to our Bison Wind Energy Center.