Income Tax Expense (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Current Income Tax Expense (Benefit) [Abstract]
Federal	[1]	$ 0.0	$ 0.0	$ 0.0	$ 0.0
State	[1]	0.1	0.1	0.1	0.1
Total Current Income Tax Expense		0.1	0.1	0.1	0.1
Deferred Income Tax Expense (Benefit) [Abstract]
Federal	[2]	(5.8)	(8.3)	(26.3)	(34.5)
State		0.9	2.9	7.3	7.1
Investment Tax Credit Amortization		(0.1)	(0.2)	(0.4)	(0.5)
Total Deferred Income Tax Expense (Benefit)		(5.0)	(5.6)	(19.4)	(27.9)
Total Income Tax Expense (Benefit)		(4.9)	(5.5)	(19.3)	(27.8)
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Income Taxes		$ 19.2	$ 33.3	$ 69.9	$ 92.4
Statutory Federal Income Tax Rate		21.00%	21.00%	21.00%	21.00%
Income Taxes Computed at Statutory Federal Rate		$ 4.0	$ 7.0	$ 14.7	$ 19.4
Increase (Decrease) in Income Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit		0.7	2.4	5.8	5.7
Production Tax Credits		(9.5)	(12.5)	(36.6)	(45.9)
Regulatory Differences – Excess Deferred Tax		(2.1)	(3.2)	(6.7)	(8.5)
Non-Controlling Interest in Subsidiaries		0.8	0.4	3.9	1.5
Share-Based Compensation		0.0	0.0	0.5	(0.1)
Other		1.2	0.4	(0.9)	0.1
Total Income Tax Expense (Benefit)		(4.9)	$ (5.5)	$ (19.3)	$ (27.8)
Effective Tax Rate				(27.60%)	(30.10%)
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits		1.5		$ 1.5		$ 1.4
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate		$ 0.6		$ 0.6

[1] For each of the three and nine months ended September 30, 2021 and 2020, the federal and state current tax expense was minimal due to NOLs which resulted from the bonus depreciation provisions of certain tax legislation. Federal and state NOLs are being carried forward to offset current and future taxable income.

[2] For each of the three and nine months ended September 30, 2021 and 2020, the federal income tax benefit is primarily due to production tax credits.