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Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets $ 18.5 [1] $ 23.0 [1]
Non-Current Regulatory Assets 277.1 263.8
Total Regulatory Assets 295.6 286.8
Non-Current Regulatory Liabilities 103.1 81.0
Regulatory Assets and Liabilities Currently Earning a Return No regulatory assets or liabilities are currently earning a return.  
Income Taxes [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 17.6 17.0
Plant Removal Obligations [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 22.7 19.7
Wholesale and Retail Contra AFUDC [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 37.6 19.7
Deferred Fuel [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Total Current Regulatory Assets 18.5 [1] 23.0 [1]
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 158.6 [2] 164.1 [2]
Non-Current Regulatory Liabilities 15.1 [2] 16.3 [2]
Income Taxes [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 37.9 35.3
Asset Retirement Obligations [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 17.2 16.0
Cost Recovery Riders [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 54.5 [3] 39.6 [3]
PPACA Income Tax Deferral [Member]
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 5.0 5.0
Other
    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 3.9 3.8
Non-Current Regulatory Liabilities $ 10.1 $ 8.3
[1] Current regulatory assets are included in Prepayments and Other on the Consolidated Balance Sheet.
[2] Defined benefit pension and other postretirement items included in our Regulated Operations, which are otherwise required to be recognized in accumulated other comprehensive income, are recognized as regulatory assets or regulatory liabilities on the Consolidated Balance Sheet. (See Note 14. Pension and Other Postretirement Benefit Plans.)
[3] The cost recovery rider regulatory asset is primarily due to capital expenditures related to our Bison Wind Energy Center and is recognized in accordance with the accounting standards for alternative revenue programs.