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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Jun. 30, 2020
Debt Instrument [Line Items]      
Short-Term Borrowings Outstanding $ 29.7 $ 54.7 $ 74.9
Revolving Credit Facility      
Debt Instrument [Line Items]      
Revolving credit facility, limit 120.0 120.0 120.0
Short-Term Borrowings Outstanding 29.7 54.7 74.9
Letter of Credit Outstanding   0.1 0.1
Available revolving credit facility $ 90.3 $ 65.2 $ 45.0