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Share-based compensation - Schedule of Restricted Share Units and Restricted Stock Activity (Detail) - Restricted Stock Units and Restricted Stock [Member] - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of units, Beginning Balance 12,407,029 13,721,000 2,960,000
Number of units, Additions as a result of the reverse acquisition     416,800
Number of units, Granted 2,732,000 1,339,758 13,890,500
Number of units, Vested and delivered (3,659,520) (1,074,327) (382,300)
Number of units, Withheld as treasury stock [1] (192,288) (360,235)  
Number of units, Vested not delivered (2,568,318) [2] (1,022,667) [3] (3,085,000) [3]
Number of units, Forfeited (567,998) (196,500) (79,000)
Number of units, Ending Balance 8,150,905 [2] 12,407,029 13,721,000
Weighted average grant-date fair value, Beginning Balance $ 8.40 $ 7.78 $ 2.00
Weighted average grant-date fair value, Additions as a result of the reverse acquisition     4.81
Weighted average grant-date fair value, Granted 5.61 4.17 9.16
Weighted average grant-date fair value, Vested and delivered 8.29 3.28 5.55 [3]
Weighted average grant-date fair value, Withheld as treasury stock [1] 8.48 1.52  
Weighted average grant-date fair value, Vested not delivered 3.43 [3] 2.59 [3] 8.36
Weighted average grant-date fair value, Forfeited 5.65 7.16 5.78
Weighted average grant-date fair value, Ending Balance $ 9.27 [2] $ 8.40 $ 7.78
[1] As discussed in Note 13 above, the increase in treasury stock was due to shares withheld to pay statutory withholding taxes upon the vesting of RSUs during related periods, among which, 200,000 shares of treasury stock were subsequently sold to an investor in 2016. As of December 31, 2017, the cumulative shares withheld as treasury stock were 352,523.
[2] Among the unvested shares as of December 31, 2017, there were 150,000 shares of unvested restricted stock, which were included in the issued and outstanding shares of the Company’s common stock as of December 31, 2017.
[3] Vested not delivered represent the vested RSUs with deferred delivery at a future time. As of December 31, 2017, the cumulative shares of RSUs included in “vested not delivered” were 6,675,985.