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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Debt Instrument [Line Items]      
Short-Term Borrowings Outstanding $ 111,900,000 $ 81,900,000 $ 36,900,000
Revolving Credit Facility      
Debt Instrument [Line Items]      
Revolving credit facility, limit 120,000,000 120,000,000 120,000,000
Short-Term Borrowings Outstanding 111,900,000 81,900,000 36,900,000
Letters of Credit Outstanding 1,100,000 1,100,000 1,100,000
Available revolving credit facility $ 7,000,000 $ 37,000,000 $ 82,000,000