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Significant Accounting Policies - Computation Of Basic And Diluted Net Income (Loss) Per Share (Parenthetical) (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Abstract]      
Options to purchase shares of Common Stock not included in the computation of diluted income (loss) per share 53,913 1,696,222 238,792