Income Tax Expense (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current Tax Expense [Abstract]
Federal	[1]	$ 0.0	$ 1.1	$ 0.0
State	[1]	0.2	2.9	0.1
Total Current Tax Expense		0.2	4.0	0.1
Deferred Tax Expense [Abstract]
Federal		19.4	25.3	22.9
State		6.5	8.2	6.5
Investment Tax Credit Amortization		(0.8)	(0.8)	(0.8)
Total Deferred Tax Expense		25.1	32.7	28.6
Total Income Tax Expense		$ 25.3	$ 36.7	$ 28.7

[1] For the years ended December 31, 2015, 2014, and 2013, the federal and state current tax expense was minimal due to NOLs which resulted from the bonus depreciation provisions of the Protecting Americans from Tax Hikes Act of 2015, the Tax Increase Prevention Act of 2014 and the American Taxpayer Relief Act of 2012. The federal and state NOLs will be carried forward to offset future taxable income. The year ended December 31, 2014 includes the resolution of an Internal Revenue Service examination for tax years 2005 through 2009 and the impacts of initiatives implemented on the 2013 federal and state tax returns to utilize tax carryforwards that may have expired.