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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Short-Term Borrowings Outstanding $ 54,700,000 $ 58,600,000
Revolving Credit Facility    
Debt Instrument [Line Items]    
Revolving credit facility, limit 120,000,000.0 120,000,000.0
Short-Term Borrowings Outstanding 54,700,000 58,600,000
Letters of Credit Outstanding 100,000 100,000
Available revolving credit facility $ 65,200,000 $ 61,300,000