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Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets $ 18.3 $ 22.5
Non-Current Regulatory Assets 337.5 340.3
Total Regulatory Assets 355.8 362.8
Non-Current Regulatory Liabilities 63.1 60.1
Income Taxes [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 17.7 19.5
Plant Removal Obligations [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 19.3 18.1
Wholesale and Retail Contra AFUDC [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 16.7 15.5
Conservation Improvement Program [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 2.7 0
Other [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 6.7 7.0
Deferred Fuel [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets 18.3 [1] 22.5 [1]
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 251.5 260.7
Income Taxes [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 34.3 36.0
Asset Retirement Obligation [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 13.9 12.1
Cost Recovery Riders [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 29.2 [2] 18.5 [2]
PPACA Income Tax Deferral [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 5.0 5.0
Conservation Improvement Program [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 0 4.3
Other [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets $ 3.6 $ 3.7
[1] Current regulatory assets are included in prepayments and other on the Consolidated Balance Sheet.
[2] The cost recovery rider regulatory asset is primarily due to capital expenditures related to Bison.