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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Short-Term Borrowings Outstanding $ 58,600,000 $ 82,800,000
Revolving Credit Facility    
Debt Instrument [Line Items]    
Revolving credit facility, limit 120,000,000.0 120,000,000.0
Short-Term Borrowings Outstanding 58,600,000 82,800,000
Letters of Credit Outstanding 100,000  
Available revolving credit facility $ 61,300,000 $ 37,200,000