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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Sep. 30, 2019
Dec. 31, 2018
Sep. 30, 2018
Debt Instrument [Line Items]      
Short-Term Borrowings Outstanding $ 49,200,000 $ 82,800,000 $ 69,100,000
Revolving Credit Facility      
Debt Instrument [Line Items]      
Revolving credit facility, limit 120,000,000.0 120,000,000.0 120,000,000.0
Short-Term Borrowings Outstanding 49,200,000 82,800,000 69,100,000
Letters of Credit Outstanding 100,000    
Available revolving credit facility $ 70,700,000 $ 37,200,000 $ 50,900,000