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Summary of Gross Realized Gain and Loss from Sale of Available-for-Sale Marketable Securities (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]      
Gross realized gains from sale of available-for-sale marketable securities $ 24 $ 4 $ 99
Gross realized losses from sale of available-for-sale marketable securities $ (33) $ (82) $ (93)