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Class A Redeemable Convertible Preferred Stock - Schedule of Beneficial Conversion Feature At Original Issue Date (Detail) - Series A Redeemable Convertible Preferred Stock [Member] - USD ($)
$ / shares in Units, $ in Thousands
Dec. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Temporary Equity [Line Items]        
Common shares the stock coverts into 14,666,667      
Effective conversion price $ 4.99      
Stock price on issue date 8.04      
Excess fair value over conversion price $ 3.05   $ 3.05  
Temporary equity, fair value $ 73,200      
Value of Beneficial Conversion Feature (44,720) $ (44,700)   $ (44,720)
Carrying amount of preferred stock $ 28,480   $ 12,816 $ 28,480