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Share-based payments - Schedule of Stock Options Activity (Detail) - $ / shares
 6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Number of options, Beginning Balance 352,000  
Number of options, Ending Balance 352,000 352,000
Options vested and expected to vest as of June 30, 2017 352,000  
Number of options, Options Exercisable 294,500  
Weighted average exercise price per share, Beginning Balance $ 10.25  
Weighted average exercise price per share, Ending Balance 10.25 $ 10.25
Options vested and expected to vest as of June 30, 2017 10.25  
Weighted average exercise price per share, Options Exercisable $ 8.36  
Weighted average remaining contractual term, Options outstanding 3 years 10 months 24 days 4 years 4 months 24 days
Options vested and expected to vest as of June 30, 2017 3 years 10 months 24 days  
Weighted average remaining contractual term, Options exercisable 3 years