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Operations and Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Inventories [Abstract]          
Fuel $ 27.5   $ 27.5   $ 28.6
Materials and Supplies 41.8   41.8   40.5
Total Inventories 69.3   69.3   69.1
Prepayments and Other Current Assets [Abstract]          
Deferred Fuel Adjustment Clause 18.5   18.5   17.5
Other 7.0   7.0   9.6
Total Prepayments and Other Current Assets 25.5   25.5   27.1
Other Current Liabilites [Abstract]          
Customer Deposits 25.8   25.8   16.3
Other 25.5   25.5   29.3
Total Other Current Liabilities 51.3   51.3   45.6
Other Non-Current Liabilities [Abstract]          
Asset Retirement Obligation 61.0   61.0   57.0
Other 49.6   49.6   48.1
Total Other Non-Current Liabilities 110.6   110.6   105.1
Supplemental Statement of Cash Flows Information [Abstract]          
Cash Paid During the Period for Interest - Net of Amounts Capitalized     21.7 21.9  
Cash Paid During the Period for Income Taxes     0.2 0.4  
Noncash Investing and Financing Activities [Abstract]          
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment     14.8 (13.2)  
AFUDC - Equity 1.2 0.5 1.9 1.1  
Accounts Receivable [Abstract]          
Pre-Bankruptcy Petition Receivable $ 3.2   $ 3.2