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Operations and Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Inventories [Abstract]      
Fuel $ 31.8   $ 28.6
Materials and Supplies 40.4   40.5
Total Inventories 72.2   69.1
Prepayments and Other Current Assets [Abstract]      
Deferred Fuel Adjustment Clause 15.5   17.5
Other 7.4   9.6
Total Prepayments and Other Current Assets 22.9   27.1
Other Current Liabilites [Abstract]      
Customer Deposits 21.3   16.3
Other 20.5   29.3
Total Other Current Liabilities 41.8   45.6
Other Non-Current Liabilities [Abstract]      
Asset Retirement Obligation 60.2   57.0
Other 50.9   48.1
Total Other Non-Current Liabilities 111.1   105.1
Supplemental Statement of Cash Flows Information [Abstract]      
Cash Paid During the Period for Interest - Net of Amounts Capitalized 12.3 10.4  
Cash Paid During the Period for Income Taxes 0.2 0.2  
Noncash Investing and Financing Activities [Abstract]      
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment (12.3) (14.4)  
AFUDC - Equity 0.8 0.6  
Accounts Receivable [Abstract]      
Pre-Bankruptcy Petition Receivable $ 3.2