Income Tax Expense (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Current Tax Expense [Abstract]
Federal	[1]	$ 0.0	$ 0.0	$ 0.0	$ 0.0
State	[1],[2]	0.2	1.8	0.5	1.9
Total Current Tax Expense		0.2	1.8	0.5	1.9
Deferred Tax Expense (Benefit) [Abstract]
Federal		14.8	11.2	23.5	20.4
State		(0.4)	0.7	3.6	5.4
Investment Tax Credit Amortization		(0.2)	(0.3)	(0.6)	(0.6)
Total Deferred Tax Expense		14.2	11.6	26.5	25.2
Total Income Tax Expense		14.4	13.4	$ 27.0	$ 27.1
Effective Tax Rate				18.00%	22.70%
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		74.7	55.0	$ 149.7	$ 119.4
Statutory Federal Income Tax Rate				35.00%	35.00%
Income Taxes Computed at 35 percent Statutory Federal Rate				$ 52.4	$ 41.8
Increase (Decrease) in Tax [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit				2.6	4.8
Production Tax Credits				(27.0)	(16.4)
Regulatory Differences for Utility Plant				(0.7)	(2.1)
Other				(0.3)	(1.0)
Total Income Tax Expense		14.4	$ 13.4	27.0	$ 27.1
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits		2.9		2.9		$ 2.0
Unrecognized Tax Benefits That Would Favorably Impact Effective Tax Rate		$ 0.4		$ 0.4

[1] For the quarter and nine months ended September 30, 2015, 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.

[2] For the quarter and nine months ended September 30, 2014, the state current tax expense reflected initiatives implemented on the 2013 federal and state tax returns to utilize tax carryforwards that may have expired due to NOL carryforwards from prior periods. State NOL and alternative minimum tax carryforwards remaining after utilization in 2015 will be carried forward to offset future income.