Income Tax Expense (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Current Tax Expense [Abstract]
Federal	[1]	$ 0.0	$ 0.0	$ 0.0	$ 0.0
State	[1]	0.2	0.1	0.3	0.1
Total Current Tax Expense		0.2	0.1	0.3	0.1
Deferred Tax Expense [Abstract]
Federal		3.9	2.9	8.7	9.2
State		2.5	2.0	4.0	4.7
Investment Tax Credit Amortization		(0.2)	(0.1)	(0.4)	(0.3)
Total Deferred Tax Expense		6.2	4.8	12.3	13.6
Total Income Tax Expense		6.4	4.9	$ 12.6	$ 13.7
Effective Tax Rate				16.80%	21.30%
Combined Federal and State Statutory Tax Rate				41.00%
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes		28.7	21.8	$ 75.0	$ 64.4
Statutory Federal Income Tax Rate				35.00%	35.00%
Income Taxes Computed at 35 percent Statutory Federal Rate				$ 26.3	$ 22.5
Increase (Decrease) in Tax [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit				2.8	3.1
Production Tax Credits				(19.2)	(12.8)
Regulatory Differences for Utility Plant				(0.4)	(1.6)
Other				3.1	2.5
Total Income Tax Expense		6.4	$ 4.9	12.6	$ 13.7
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits		2.0		2.0		$ 2.0
Unrecognized Tax Benefits That Would Favorably Impact Effective Tax Rate		$ 0.3		$ 0.3

[1] For the six months ended June 30, 2015 and 2014, the federal and state current tax expense was minimal due to the utilization of NOL carryforwards from prior periods. The NOL carryforwards resulted from the bonus depreciation provisions of the Tax Increase Prevention Act of 2014 and the American Taxpayer Relief Act of 2012.