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Share-based compensation - Summary of Stock Option Activity (Detail) - Share options - 2010 Plan - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Number of share options        
Outstanding at beginning of period 16,500 389,815 1,493,470  
Vested and expected to vest 10,000 16,500 389,693 1,440,923
Exercisable 10,000 16,500 389,190 1,217,050
Forfeited     (109,925)  
Expired (6,500) (373,315) (989,730)  
Exercised     (4,000)  
Outstanding at end of period 10,000 16,500 389,815 1,493,470
Weighted average exercise price (USD)        
Outstanding at beginning of period $ 3.97 $ 3.90 $ 2.65  
Vested and expected to vest 3.97 3.97 3.90 $ 2.67
Exercisable 3.97 3.97 3.90 2.70
Forfeited     2.89  
Expired 3.97 3.89 2.28  
Exercised     0.83  
Outstanding at end of period 3.97 3.97 3.90 2.65
Weighted average grant-date fair value (USD)        
Vested and expected to vest 1.01 1.56 0.95 0.85
Exercisable $ 1.01 $ 1.56 $ 0.95 $ 0.84
Share option activity - additional disclosures        
Weighted-average remaining contractual life, vested and expected to vest 1 year 1 month 28 days 1 year 4 months 13 days 1 year 7 months 21 days 3 years 2 months 27 days
Weighted-average remaining contractual life, exercisable 1 year 1 month 28 days 1 year 4 months 13 days 1 year 7 months 21 days 3 years 2 months 12 days
Weighted-average remaining contractual life, outstanding 1 year 1 month 28 days 1 year 4 months 13 days 1 year 7 months 21 days 3 years 4 months 21 days
Aggregate intrinsic value, vested and expected to vest $ 0 $ 0 $ 0 $ 6
Aggregate intrinsic value, exercisable 0 0 0 6
Aggregate intrinsic value, outstanding $ 0 $ 0 $ 0 $ 6