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Share-based compensation - Weighted Average Contractual Life and Exercise Price (Detail) - Stock Option Plan - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Number of Options    
Options outstanding 4,415,397 3,069,440
Options vested 2,258,338 2,977,440
Options unvested 2,157,059 92,000
Weighted Average Remaining Contractual Life    
Options outstanding 2 years 8 months 12 days 2 years 4 months 24 days
Options vested 2 years 7 months 6 days 2 years 4 months 24 days
Options unvested 2 years 10 months 24 days 3 years
Weighted Average Exercise Price    
Options outstanding $ 1.20 $ 4.28
Options vested 1.29 3.94
Options unvested $ 1.10 $ 5.76