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Share-based compensation - Schedule of Stock Options Activity (Detail) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Number of options, Beginning Balance 222,000  
Number of options, Ending Balance 222,000 222,000
Options vested and expected to vest as of March 31, 2018 222,000  
Options exercisable as of March 31, 2018 222,000  
Weighted average exercise price per share, Beginning Balance $ 12.59  
Weighted average exercise price per share, Ending Balance 12.59 $ 12.59
Options vested and expected to vest as of March 31, 2018 12.59  
Weighted average exercise price per share, Options Exercisable as of March 31, 2018 $ 12.59  
Weighted average remaining contractual term, Options outstanding 1 year 10 months 24 days 5 years 4 months 24 days
Options vested and expected to vest as of March 31, 2018 1 year 10 months 24 days  
Weighted average remaining contractual term, Options exercisable as of March 31, 2018 1 year 10 months 24 days