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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 |  |

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