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Share-based payments - Unvested Balance of Options (Detail) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Number of options    
Unvested as of December 31, 2015 126,667  
Vested (30,000)  
Forfeited (30,000)  
Unvested as of March 31, 2016 66,667 126,667
Weighted average exercise price per share    
Unvested as of December 31, 2015 $ 9.02  
Vested 5.50  
Forfeited 9.59  
Unvested as of March 31, 2016 $ 10.34 $ 9.02
Weighted average remaining contractual term    
Unvested 9 years 4 months 24 days 8 years 1 month 6 days