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Share-based compensation - Schedule of Stock Options Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Number of options    
Beginning balance (in shares) 112,000  
Granted (in shares) 2,064,000  
Forfeited (in shares) (56,000)  
Ending balance (in shares) 2,120,000 112,000
Options exercisable (in shares) 112,000  
Weighted average exercise price per share    
Beginning balance (in usd per share) $ 13.98  
Granted (in usd per share) 4.72  
Forfeited (in usd per share) 4.72  
Ending balance (in usd per share) 5.20 $ 13.98
Options exercisable (in usd per share) $ 13.98  
Weighted average remaining contractual term    
Outstanding 9 years 2 months 2 years 10 months
Granted 9 years 7 months  
Options exercisable as of June 30, 2019 2 years 4 months  
Aggregate intrinsic value    
Outstanding $ 1,303 $ 0
Options exercisable as of June 30, 2019 $ 5