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Share-based compensation - Unvested Balance of Options (Detail) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Number of options    
Unvested, Beginning balance 47,500  
Vested [1] (47,500)  
Unvested, Ending balance   47,500
Weighted average exercise price per share    
Unvested, Beginning balance $ 8.96  
Vested [1] $ 8.96  
Unvested, Ending balance   $ 8.96
Weighted average remaining contractual term    
Unvested   7 years 10 months 24 days
[1] As discussed in “Spin-off of red violet” above, the vesting of 47,500 shares of stock options was accelerated as a result of the Spin-off of red violet.