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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Short-Term Borrowings Outstanding $ 82,800,000 $ 38,300,000
Revolving Credit Facility    
Debt Instrument [Line Items]    
Revolving credit facility, limit 120,000,000 120,000,000
Short-Term Borrowings Outstanding 82,800,000 38,300,000
Letters of Credit Outstanding 0 0
Available revolving credit facility $ 37,200,000 $ 81,700,000