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Operations and Significant Accounting Policies (Details) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Sep. 30, 2011
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Customer Information [Line Items]        
Escrow Deposit   $ 5.4    
Non-Controlling Interest in Subsidiaries [Abstract]        
Purchase of Non-Controlling Interest at Book Value 8.8      
Purchase of Non-Controlling Interest, Shares Issued 200      
Trade Accounts Receivable [Abstract]        
Billed   78.7 70.4  
Unbilled   18.7 17.4  
Less: Allowance for Doubtful Accounts   1.1 1.0  
Total Trade Accounts Receivable   96.3 86.8  
Income Taxes Receivable   0 2.2  
Total Accounts Receivable - Net   96.3 89.0  
Inventories [Abstract]        
Fuel   13.1 [1] 28.0 [1]  
Materials and Supplies   46.2 41.8  
Total Inventories   59.3 69.8  
Impairement of Long-Lived Assets [Abstract]        
Land Impairment   0 0 1.7
Prepayments and Other Current Assets [Abstract]        
Deferred Fuel Adjustment Clause   23.0 22.5  
Other   12.1 11.1  
Total Prepayments and Other Current Assets   35.1 33.6  
Other Current Liabilities [Abstract]        
Customer Deposits   26.0 28.8  
Other   26.6 33.8  
Total Other Current Liabilities   52.6 62.6  
Other Non-Current Liabilities [Abstract]        
Asset Retirement Obligation   81.8 77.9 57.0
Other   45.4 45.4  
Total Other Non-Current Liabilities   $ 127.2 $ 123.3  
Income Tax Disclosure [Abstract]        
More Likely Than Not Definition   50.00%    
Municipal Customers [Member]
        
Customer Information [Line Items]        
Number of Customers   16    
Non-Affiliated Private Utility [Member]
        
Customer Information [Line Items]        
Number of Customers   1    
Retail Customers [Member]
        
Customer Information [Line Items]        
Number of Customers   143,000    
Electric [Member]
        
Customer Information [Line Items]        
Number of Customers   15,000    
Natural Gas [Member]
        
Customer Information [Line Items]        
Number of Customers   12,000    
Water [Member]
        
Customer Information [Line Items]        
Number of Customers   10,000    
[1] Fuel inventory was lower in 2013 primarily due to higher than expected thermal generation and the timing of coal shipments.