Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Inventories [Abstract]
Fuel	$ 31.3		$ 31.3		$ 28.6
Materials and Supplies	45.4		45.4		40.5
Total Inventories	76.7		76.7		69.1
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause	18.2		18.2		17.5
Other	5.6		5.6		9.6
Total Prepayments and Other Current Assets	23.8		23.8		27.1
Other Current Liabilites [Abstract]
Customer Deposits	28.8		28.8		16.3
Other	24.3		24.3		29.3
Total Other Current Liabilities	53.1		53.1		45.6
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	61.7		61.7		57.0
Other	47.7		47.7		48.1
Total Other Non-Current Liabilities	109.4		109.4		105.1
Supplemental Statement of Cash Flows Information [Abstract]
Cash Paid During the Period for Interest - Net of Amounts Capitalized			32.3	32.4
Cash Paid (Received) During the Period for Income Taxes			0.2	(11.1)
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment			(13.1)	(14.8)
AFUDC - Equity	1.5	0.6	3.4	1.7
Accounts Receivable [Abstract]
Pre-Bankruptcy Petition Receivable	$ 3.2		$ 3.2