Income Tax Expense (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
Current Tax Expense (Benefit) [Abstract]
Federal	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]
State	(0.1)	[1]	0	[1]	(0.1)	[1]	0	[1]
Total Current Tax Expense (Benefit)	(0.1)		0		(0.1)		0
Deferred Tax Expense (Benefit) [Abstract]
Federal	7.3		10.1		15.5		23.3
State	1.7	[2]	0.4	[2]	5.5	[2]	0.7	[2]
Investment Tax Credit Amortization	(0.2)		(0.2)		(0.6)		(0.6)
Total Deferred Tax Expense	8.8		10.3		20.4		23.4
Total Income Tax Expense	8.7		10.3		20.3		23.4
Effective Income Tax Rate					22.10%		25.50%
Federal Statutory Income Tax Rate					41.00%
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits	3.2				3.2				2.7
Unrecognized Tax Benefits that Would Favorably Impact Effective Tax Rate	$ 0.8				$ 0.8

[1] We incurred net operating losses (NOLs) due to the bonus depreciation provisions of the American Taxpayer Relief Act of 2012 and the Tax Relief Unemployment Insurance Reauthorization and Job Creation Act of 2010 for the quarter and nine months ended September 30, 2013 and 2012. The 2013 and 2012 federal and state NOLs will be carried forward to offset future taxable income.

[2] The quarter and nine months ended September 30, 2012 reflected increased state tax benefit from state renewable tax credits compared to the quarter and nine months ended September 30, 2013.