Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Inventories [Abstract]
Fuel	$ 18.3		$ 18.3		$ 28.0
Materials and Supplies	46.4		46.4		41.8
Total Inventories	64.7		64.7		69.8
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause	22.8		22.8		22.5
Other	6.4		6.4		11.1
Total Prepayments and Other Current Assets	29.2		29.2		33.6
Other Current Liabilites [Abstract]
Customer Deposits	27.5		27.5		28.8
Other	31.8		31.8		33.8
Total Other Current Liabilities	59.3		59.3		62.6
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	82.6		82.6		77.9
Other	46.5		46.5		45.4
Total Other Non-Current Liabilities	129.1		129.1		123.3
Supplemental Statement of Cash Flows Information [Abstract]
Cash Paid During the Period for Interest - Net of Amounts Capitalized			36.0	32.3
Cash Paid During the Period for Income Taxes			0.6	0.2
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment			(10.0)	(13.1)
Capitalized Asset Retirement Costs			1.9	2.4
AFUDC - Equity	$ 1.2	$ 1.5	$ 3.4	$ 3.4