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Summary of Interest Expense and Interest Income (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Interest Expense      
Long-Term Debt $ 24.7 $ 26.0 $ 24.8
Short-Term Debt 3.0 0.8 1.4
Regulatory Liabilities 0.6 0.4 0.2
Subtotal Interest Expense 28.3 27.2 26.4
Interest Income      
Interest and Other Income (2.8) (1.6) (2.6)
Interest Expense, Net 25.5 25.6 23.8
Regulatory Assets      
Interest Income      
Interest and Other Income (1.0) (0.5) (0.8)
AFUDC and Other      
Interest Income      
Interest and Other Income $ (1.8) $ (1.1) $ (1.8)