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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, 2012
Dec. 31, 2011
Dec. 31, 2010
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   70.4 63.7  
Unbilled   17.4 15.6  
Less: Allowance for Doubtful Accounts   1.0 0.9  
Total Trade Accounts Receivable   86.8 78.4  
Income Taxes Receivable   2.2 1.3  
Total Accounts Receivable - Net   89.0 79.7  
Inventories [Abstract]        
Fuel   28.0 28.6  
Materials and Supplies   41.8 40.5  
Total Inventories   69.8 69.1  
Impairement of Long-Lived Assets [Abstract]        
Land Impairment   0 1.7  
Prepayments and Other Current Assets [Abstract]        
Deferred Fuel Adjustment Clause   22.5 17.5  
Other   11.1 9.6  
Total Prepayments and Other Current Assets   33.6 27.1  
Other Current Liabilities [Abstract]        
Customer Deposits   28.8 [1] 16.3 [1]  
Other   33.8 29.3  
Total Other Current Liabilities   62.6 45.6  
Other Non-Current Liabilities [Abstract]        
Asset Retirement Obligation   77.9 57.0 50.3
Other   45.4 48.1  
Total Other Non-Current Liabilities   $ 123.3 $ 105.1  
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] 2012 primarily due to customer security deposits for capital expenditures relating to a transmission project.