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Share-based compensation - Weighted Average Contractual Life and Exercise Price (Detail)
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Number of Options  
Options outstanding and vested | shares 2,840,848
Weighted Average Remaining Contractual Life  
Weighted Average Remaining Contractual Life Options outstanding and vested 2 years 1 month 20 days
Weighted Average Exercise Price  
Weighted Average Exercise Price, Options outstanding and vested | $ / shares $ 4.14