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Debt and Financing Arrangements - Summary of Interest Expense and Interest Income (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Interest Expense      
Long-Term Debt $ 27.8 $ 24.7 $ 24.7
Short-Term Debt 8.4 9.2 3.0
Regulatory Liabilities 1.5 1.0 0.6
Subtotal Interest Expense 37.7 34.9 28.3
Interest Income      
Interest and Other Income (8.4) (6.2) (2.8)
Interest Expense, Net 29.3 28.7 25.5
Regulatory Assets      
Interest Income      
Interest and Other Income (4.1) (3.3) (1.0)
AFUDC and Other      
Interest Income      
Interest and Other Income $ (4.3) $ (2.9) $ (1.8)