Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2013
Current Tax Expense (Benefit) [Abstract]
Federal	$ 0	[1]	$ 0.2	[1]
State	0	[1]	0	[1]
Total Current Tax Expense (Benefit)	0		0.2
Deferred Tax Expense (Benefit) [Abstract]
Federal	6.3		5.8
State	2.7		1.7
Investment Tax Credit Amortization	(0.2)		(0.2)
Total Deferred Tax Expense	8.8		7.3
Total Income Tax Expense	8.8		7.5
Effective Income Tax Rate	20.70%		18.80%
Federal Statutory Income Tax Rate	41.00%
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits	1.2				1.2
Unrecognized Tax Benefits that Would Favorably Impact Effective Tax Rate	$ 0.2

[1] For the quarter ended March 31, 2014, the federal and state current tax expense of zero was due to the utilization of NOL carryforwards from prior periods. For the quarter ended March 31, 2013, the federal and state current tax expense of $0.2 million and zero, respectively, was due to federal and state NOLs which resulted from the bonus depreciation provision of the American Taxpayer Relief Act of 2012. The federal and state NOLs remaining after utilization in 2014 will be carried forward to offset future taxable income.