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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Debt Instrument [Line Items]      
Short-Term Borrowings Outstanding $ 65,800,000 $ 82,800,000 $ 45,300,000
Revolving Credit Facility      
Debt Instrument [Line Items]      
Revolving credit facility, limit 120,000,000 120,000,000 120,000,000
Short-Term Borrowings Outstanding 65,800,000 82,800,000 45,300,000
Available revolving credit facility $ 54,200,000 $ 37,200,000 $ 74,700,000