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Share-based payments - Schedule of Stock Options Activity (Detail) - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Number of options, Beginning Balance 462,000  
Number of options, Granted 30,000  
Number of options, Forfeited (30,000)  
Number of options, Expired (100,000)  
Number of options, Ending Balance 362,000 462,000
Number of options, Options Exercisable 271,583  
Weighted average exercise price per share, Beginning Balance $ 9.52  
Weighted average exercise price per share, Granted 5.04  
Weighted average exercise price per share, Forfeited 9.59  
Weighted average exercise price per share, Expired 6.02  
Weighted average exercise price per share, Ending Balance 10.10 $ 9.52
Weighted average exercise price per share, Options Exercisable $ 10.60  
Weighted average remaining contractual term 4 years 9 months 18 days 5 years 3 months 18 days
Weighted average remaining contractual term, Options exercisable 3 years 3 months 18 days