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Share-based compensation - Schedule of Restricted Share Units and Restricted Stock Activity (Footnote) (Detail) - shares
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Treasury stock, shares (in shares)   1,233,198 352,523 160,235
Diluted (in shares)   76,705,877 61,153,069  
Restricted Stock Units Or Common Stock Grants        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Decrease in equity instruments other than options vested and unissued in prior period (in shares) 3,766,068      
Increase in equity instruments other than options vested subject to deferred delivery terms (in shares)     2,568,318  
Diluted (in shares) 2,909,917   6,675,985  
Restricted Stock Units Common Stock Grants And Restricted Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Equity instruments other than options vested but deferred in prior period (in shares) 6,278,318   34,584  
Restricted Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of restricted stock units outstanding (in shares)   150,000