Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Income Taxes	$ 41.0	$ 39.1
Income Taxes Computed at Statutory Federal Rate	8.6	8.2
Increase (Decrease) in Income Tax Due to:
State Income Taxes – Net of Federal Income Tax Benefit	4.6	3.4
Production tax credits	(11.6)	(10.4)
Investment tax credits	(0.3)	(2.2)
Regulatory Differences – Excess Deferred Tax	(3.5)	(2.8)
Non-Controlling Interest in Subsidiaries	2.8	3.8
AFUDC – Equity	(0.6)	(0.3)
Other	4.0	1.8
Total Income Tax Expense	$ 4.0	$ 1.5