Income Tax Expense (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Current Income Tax Expense (Benefit) [Abstract]
Federal	[1]	$ 0.0	$ 0.0	$ 0.0	$ 0.0
State	[1]	0.0	0.1	0.4	0.3
Total Current Income Tax Expense (Benefit)		0.0	0.1	0.4	0.3
Deferred Income Tax Expense (Benefit) [Abstract]
Federal	[2]	(2.2)	9.2	(16.4)	20.3
State		2.8	5.0	7.8	14.5
Investment Tax Credit Amortization		(0.2)	(0.1)	(0.5)	(0.5)
Total Deferred Income Tax Expense (Benefit)		0.4	14.1	(9.1)	34.3
Total Income Tax Expense (Benefit)		0.4	14.2	(8.7)	34.6
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes		$ 31.1	$ 59.1	$ 104.3	$ 165.4
Statutory Federal Income Tax Rate		21.00%	35.00%	21.00%	35.00%	35.00%
Income Taxes Computed at Statutory Federal Rate		$ 6.5	$ 20.7	$ 21.9	$ 57.9
Increase (Decrease) in Income Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit		2.2	3.3	6.4	9.6
Production Tax Credits		(9.4)	(10.4)	(35.0)	(33.4)
Regulatory Differences – Excess Deferred Tax		(1.1)	0.3	(5.5)	1.0
Other		2.2	0.3	3.5	(0.5)
Total Income Tax Expense (Benefit)		0.4	$ 14.2	$ (8.7)	$ 34.6
Effective Tax Rate				8.30%	20.90%
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits		1.7		$ 1.7		$ 1.7
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate		$ 0.8		$ 0.8

[1] For the quarter and nine months ended September 30, 2018, and 2017, 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.

[2] For the quarter and nine months ended September 30, 2018, the federal tax benefit is primarily due to the reduction of the federal statutory tax rate from 35 percent to 21 percent enacted as part of the TCJA and production tax credits.