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Available-for-Sale Marketable Securities in Unrealized Position (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value $ 44,514 $ 40,121
Less than 12 Months, Unrealized Loss (97) (24)
Total, Fair Value 44,514 40,121
Total, Unrealized Loss (97) (24)
United States government agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 8,729 6,035
Less than 12 Months, Unrealized Loss (24) (1)
Total, Fair Value 8,729 6,035
Total, Unrealized Loss (24) (1)
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 35,785 34,086
Less than 12 Months, Unrealized Loss (73) (23)
Total, Fair Value 35,785 34,086
Total, Unrealized Loss $ (73) $ (23)