Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes		$ 100.1	$ 110.9	$ 109.4
Statutory Federal Income Tax Rate		21.00%	21.00%	21.00%
Income Taxes Computed at Statutory Federal Rate		$ 21.0	$ 23.3	$ 23.0
Increase (Decrease) in Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit		8.6	8.6	6.5
Effective Income Tax Rate Reconciliation, Deferred Revaluation - Net of Federal Income Tax Benefit		(7.9)	0.0	0.0
Production Tax Credits		(50.7)	(53.5)	(62.7)
Effective income Tax Rate Reconciliation - Investment Tax Credits		(4.1)	0.0	0.0
Regulatory Differences – Excess Deferred Tax Benefit	[1]	(9.1)	(9.5)	(9.9)
Non-controlling interest		11.2	6.3	2.7
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount		(1.1)	(1.0)	(0.8)
Other		0.9	(1.1)	(2.2)
Total Income Tax Expense (Benefit)		$ (31.2)	$ (26.9)	$ (43.4)
Effective Tax Rate		(31.20%)	(24.30%)	(39.70%)

[1] Excess deferred income taxes are being returned to customers under both the Average Rate Assumption Method and amortization periods as approved by regulators. (See Note 4. Regulatory Matters.)