Income Tax Expense (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2023		Mar. 31, 2022	Dec. 31, 2022
Current Income Tax Expense (Benefit) [Abstract]
Federal	[1]	$ 5.6		$ 0.0
State	[1]	2.2		0.1
Total Current Income Tax Expense		7.8		0.1
Deferred Income Tax Expense (Benefit) [Abstract]
Federal	[2]	(8.3)		(8.6)
State		2.1		4.8
Investment Tax Credit Amortization		(0.1)		(0.2)
Total Deferred Income Tax Expense (Benefit)		(6.3)		(4.0)
Total Income Tax Expense (Benefit)		1.5		(3.9)
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Income Taxes		$ 39.1		$ 42.6
Statutory Federal Income Tax Rate		21.00%		21.00%
Income Taxes Computed at Statutory Federal Rate		$ 8.2		$ 8.9
Increase (Decrease) in Income Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit		3.4		3.9
Production Tax Credits		(10.4)	[3]	(17.6)
Effective Income Tax Rate Reconciliation, Investment Tax Credits		(2.2)	[3]	0.0
Regulatory Differences – Excess Deferred Tax		(2.8)		(3.8)
Non-Controlling Interest in Subsidiaries		3.8		3.8
Other		1.5		0.9
Total Income Tax Expense (Benefit)		$ 1.5		$ (3.9)
Effective Tax Rate		3.80%		(9.20%)
Uncertain Tax Positions [Abstract]
Gross Unrecognized Tax Benefits		$ 1.1			$ 1.3
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate		$ 0.6

[1] For the three months ended March 31, 2022, the federal and state current tax expense was minimal due to NOLs which resulted from the bonus depreciation provisions of certain tax legislation. For the three months ended March 31, 2023, the federal current tax expense was partially offset by production tax credits.

[2] For the three months ended March 31, 2023 and 2022, the federal income tax benefit is primarily due to production tax credits.

[3] For the three months ended March 31, 2023, the credits are presented net of any estimated discount on the sale of certain credits.