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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Debt Instrument [Line Items]      
Short-Term Borrowings Outstanding $ 37,400,000 $ 38,300,000 $ 79,200,000
Revolving Credit Facility      
Debt Instrument [Line Items]      
Revolving credit facility, limit 120,000,000 120,000,000 120,000,000
Short-Term Borrowings Outstanding 37,400,000 38,300,000 79,200,000
Letters of Credit Outstanding 0 0 1,100,000
Available revolving credit facility $ 82,600,000 $ 81,700,000 $ 39,700,000