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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Debt Instrument [Line Items]      
Short-Term Borrowings Outstanding $ 76,600,000 $ 81,900,000 $ 47,900,000
Revolving Credit Facility      
Debt Instrument [Line Items]      
Revolving credit facility 120,000,000 120,000,000 120,000,000
Short-Term Borrowings Outstanding 76,600,000 81,900,000 47,900,000
Letters of Credit Outstanding 1,100,000 1,100,000  
Available revolving credit facility $ 42,300,000 $ 37,000,000 $ 72,100,000