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Share-based compensation - Schedule of Stock Options Activity (Detail) - $ / shares
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Number of options, Beginning Balance 222,000  
Number of options, Expired (85,000)  
Number of options, Ending Balance 137,000 222,000
Options vested and expected to vest as of June 30, 2018 137,000  
Options exercisable as of June 30, 2018 137,000  
Weighted average exercise price per share, Beginning Balance $ 12.59  
Weighted average exercise price per share, Expired 9.15  
Weighted average exercise price per share, Ending Balance 14.72 $ 12.59
Options vested and expected to vest as of June 30, 2018 14.72  
Weighted average exercise price per share, Options Exercisable as of March 31, 2018 $ 14.72  
Weighted average remaining contractual term, Options outstanding 2 years 9 months 18 days 5 years 4 months 24 days
Options vested and expected to vest as of June 30, 2018 2 years 9 months 18 days  
Weighted average remaining contractual term, Options exercisable as of March 31, 2018 2 years 9 months 18 days