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Summary of Gross Unrealized Losses and Fair Values on Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]    
Less than 12 months, Fair value $ 48,663 $ 32,695
Available-for-sale securities- matures within one year, Gross unrealized losses (557) (389)
12 months or greater, Fair value 4,875 14,488
Available-for-sale securities- matures after one year through three years, Gross unrealized losses $ (29) $ (105)