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Summary of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 50,868 $ 54,131
Gross Unrealized Gain   3,952
Gross Unrealized Loss (97) (24)
Fair Value 50,771 58,059
Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,758 8,991
Fair Value 3,758 8,991
United States government agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 11,252 8,030
Gross Unrealized Loss (24) (1)
Fair Value 11,228 8,029
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 35,858 37,110
Gross Unrealized Loss (73) (23)
Fair Value $ 35,785 37,087
Marketable equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Gain   3,952
Fair Value   $ 3,952