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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets and Liabilities Currently Earning a Return No regulatory assets or liabilities are currently earning a return.  
Current Regulatory Assets $ 16.3 $ 16.3
Non-Current Regulatory Assets 353.3 357.3
Total Regulatory Assets 369.6 373.6
Non-Current Regulatory Liabilities 105.8 94.2
Wholesale and Retail Contra AFUDC [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 54.5 42.9
Plant Removal Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 20.9 22.8
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 12.8 13.4
Deferred Fuel [Member]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets [1] 16.3 16.3
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets [2] 212.3 223.9
Non-Current Regulatory Liabilities [2] 1.8 3.5
Cost Recovery Riders [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets [3] 64.0 59.7
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 46.9 46.6
Asset Retirement Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 20.5 17.8
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 4.6 4.3
Non-Current Regulatory Liabilities $ 15.8 $ 11.6
[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 assets are primarily due to capital expenditures related to the Bison Wind Energy Center, investment in CapX2020 projects, and the Boswell Unit 4 environmental upgrade and are recognized in accordance with the accounting standards for alternative revenue programs.