Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Income Taxes	$ 88.9	$ 19.6	$ 168.3	$ 74.7
Income Taxes Computed at Statutory Federal Rate	18.7	4.1	35.3	15.7
Increase (Decrease) in Income Tax Due to:
State Income Taxes – Net of Federal Income Tax Benefit	8.0	1.1	15.0	6.6
Deferred Revaluation – Net of Federal Income Tax Benefit	0.0	0.0	0.0	(5.2)
Production tax credits	(7.7)	(9.7)	(28.3)	(34.4)
Investment tax credits	(1.6)	(3.2)	(5.2)	(3.2)
Regulatory Differences – Excess Deferred Tax	(2.3)	(1.5)	(7.5)	(6.7)
Non-Controlling Interest in Subsidiaries	3.0	1.4	8.9	8.4
Other	1.2	0.6	2.2	(0.6)
Total Income Tax Expense (Benefit)	$ 19.3	$ (7.2)	$ 20.4	$ (19.4)