Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Current Tax Expense [Abstract]
Federal	$ 0.2	[1]	$ 0	[1]
State	0	[1]	0	[1]
Total Current Tax Expense	0.2		0
Deferred Tax Expense (Benefit) [Abstract]
Federal	5.8		8.7
State	(0.5)	[2]	(0.8)	[2]
Change in Valuation Allowance	2.2	[3]	0.6	[3]
Investment Tax Credit Amortization	(0.2)		(0.2)
Total Deferred Tax Expense	7.3		8.3
Total Income Tax Expense	7.5		8.3
Effective Income Tax Rate	18.80%		25.40%
Federal Statutory Income Tax Rate	41.00%
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits	2.6				2.7
Unrecognized Tax Benefits that Would Favorably Impact Effective Tax Rate	$ 0.7

[1] For the quarter ended March 31, 2013, the federal and state current tax expense of $0.2 million and zero, respectively, (zero and zero for the quarter ended March 31, 2012) is due to net operating losses (NOLs) which resulted primarily from the bonus depreciation provision of the American Taxpayer Relief Act of 2012 and the Tax Relief Unemployment Insurance Reauthorization and Job Creation Act of 2010. The 2013 and 2012 federal and state NOLs will be carried forward to offset future taxable income.

[2] For the quarters ended March 31, 2013 and 2012, the state deferred tax benefits of $0.5 million and $0.8 million, respectively, are primarily due to state renewable tax credits earned which will be carried forward to offset future state tax expense.

[3] For the quarters ended March 31, 2013 and 2012, the change in the valuation allowance is due to state renewable tax credits earned in 2013 and 2012 which are not expected to be utilized within their allowable tax carryforward period.