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Share-based compensation - Schedule of Restricted Share Units, Common Stock Grants and Restricted Stock Activity (Detail) - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Weighted average grant-date fair value    
Diluted (in shares) 80,063,989 71,537,743
Restricted Stock Units, Common Stock Grants and Restricted Stock    
Number of units    
Beginning balance (in shares) 3,831,965  
Granted (in shares) 1,725,930  
Vested and delivered (in shares) (755,977)  
Withheld as treasury stock (in shares) (321,631)  
Vested not delivered (in shares) (673,338)  
Forfeitures (in shares) (48,791)  
Ending balance (in shares) 3,758,158  
Weighted average grant-date fair value    
Beginning balance (in usd per share) $ 7.95  
Granted (in usd per share) 4.78  
Vested and delivered (in usd per share) 5.02  
Withheld as treasury stock (in usd per share) 5.25  
Vested not delivered (in usd per share) 7.25  
Forfeitures (in usd per share) 3.39  
Ending balance (in usd per share) $ 7.50  
Restricted Stock Units    
Number of units    
Withheld as treasury stock (in shares) (1,554,829)  
Restricted Stock Units Or Common Stock Grants    
Weighted average grant-date fair value    
Decrease in vested and unissued shares in prior period (in shares) 673,338  
Diluted (in shares) 3,583,255