Income Tax Expense (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Current Tax Expense [Abstract]
Federal	[1]	$ 0.0	$ 0.0	$ 0.0	$ 0.0
State	[1]	0.1	0.1	0.2	0.2
Total Current Tax Expense		0.1	0.1	0.2	0.2
Deferred Tax Expense [Abstract]
Federal		3.8	2.1	11.1	6.7
State		3.6	2.7	9.5	7.5
Investment Tax Credit Amortization		(0.2)	(0.2)	(0.4)	(0.4)
Total Deferred Tax Expense		7.2	4.6	20.2	13.8
Total Income Tax Expense		7.3	4.7	20.4	14.0
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes		$ 44.2	$ 29.5	$ 106.3	$ 85.2
Statutory Federal Income Tax Rate		35.00%	35.00%	35.00%	35.00%
Income Taxes Computed at 35 percent Statutory Federal Rate		$ 15.5	$ 10.3	$ 37.2	$ 29.8
Increase (Decrease) in Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit		2.4	1.8	6.3	5.0
Production Tax Credits		(10.0)	(6.6)	(23.0)	(20.5)
Other		(0.6)	(0.8)	(0.1)	(0.3)
Total Income Tax Expense		7.3	$ 4.7	$ 20.4	$ 14.0
Effective Tax Rate				19.20%	16.40%
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits		1.9		$ 1.9		$ 2.0
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate		$ 0.7		$ 0.7

[1] For the quarter and six months ended June 30, 2017, and 2016, 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.