Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes	$ 206.8	$ 100.1	$ 110.9
Income Taxes Computed at Statutory Federal Rate	43.4	21.0	23.3
Increase (Decrease) in Tax Due to:
State Income Taxes – Net of Federal Income Tax Benefit	19.7	8.6	8.6
Deferred Revaluation – Net of Federal Income Tax Benefit	0.0	(7.9)	0.0
Production Tax Credits	(31.6)	(50.7)	(53.5)
Investment Tax Credits	(5.8)	(4.1)	0.0
Regulatory Differences – Excess Deferred Tax Benefit	(9.9)	(9.1)	(9.5)
Non-Controlling Interest	13.3	11.2	6.3
AFUDC - Equity	(1.3)	(1.1)	(1.0)
Other	0.1	0.9	(1.1)
Total Income Tax Expense (Benefit)	$ 27.9	$ (31.2)	$ (26.9)