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Summary of Interest Expense and Interest Income (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Interest Expense      
Long-Term Debt $ 23.1 $ 21.8 $ 21.8
Short-Term Debt 2.6 2.5 1.4
Regulatory Liabilities 0.7 1.2 0.5
Interest Expense 26.4 25.5 23.7
Interest Income      
Interest income (2.4) (2.4) (1.2)
Total Interest Expense, net 24.0 23.1 22.5
Regulatory Assets      
Interest Income      
Interest income (0.8) (0.7) (0.3)
AFUDC and Other      
Interest Income      
Interest income [1] $ (1.6) $ (1.7) $ (0.9)
[1] AFUDC-Allowance for Funds Used During Construction