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Share-based compensation - Schedule of Restricted Share Units and Restricted Stock Activity (Parenthetical) (Detail) - shares
1 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Treasury stock, shares 160,235 352,523 160,235
Cumulative Restricted Stock Units Under Vested Not Delivered [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Cumulative restricted stock units under vested not delivered   6,675,985  
Restricted Stock Units [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Treasury stock sold to investor, shares 200,000   200,000
Restricted Stock [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of restricted stock units outstanding   150,000