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Share-based compensation - Schedule of Stock Options Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Number of options      
Number of options, beginning balance (in shares) 222,000 352,000  
Expired (in shares) (110,000)   (130,000)
Number of options, ending balance (in shares) 112,000 222,000 352,000
Options vested and expected to vest (in shares) 112,000    
Options exercisable (in shares) 112,000    
Weighted average exercise price per share      
Weighted average exercise price per share, beginning balance (in dollars per share) $ 12.59 $ 10.25  
Expired (in dollars per share) 11.17   $ 6.25
Weighted average exercise price per share, ending balance (in dollars per share) 13.98 $ 12.59 $ 10.25
Options vested and expected to vest (in dollars per share) 13.98    
Options exercisable (in dollars per share) $ 13.98    
Weighted average remaining contractual term      
Weighted average remaining contractual term, outstanding 2 years 9 months 18 days 5 years 4 months 24 days 4 years 5 months
Options vested and expected to vest 2 years 9 months 18 days    
Weighted average remaining contractual term, options exercisable 2 years 9 months 18 days    
Aggregate intrinsic value, options, outstanding beginning balance $ 0    
Aggregate intrinsic value, options, outstanding ending balance 0 $ 0  
Aggregate intrinsic value, options, vested and expected to vest 0    
Aggregate intrinsic value, options, exercisable $ 0