Debt and Financing Arrangements - Summary of Interest Expense and Interest Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Interest Expense
Long-Term Debt	$ 24.7	$ 24.7	$ 26.0
Short-Term Debt	9.2	3.0	0.8
Regulatory Liabilities	1.0	0.6	0.4
Subtotal Interest Expense	34.9	28.3	27.2
Interest Income
Interest and Other Income	(6.2)	(2.8)	(1.6)
Interest Expense, Net	28.7	25.5	25.6
Regulatory Assets
Interest Income
Interest and Other Income	(3.3)	(1.0)	(0.5)
AFUDC and Other
Interest Income
Interest and Other Income	$ (2.9)	$ (1.8)	$ (1.1)