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Share-Based Payments - Summary of Options - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Compensation costs related to awards not yet recognised $ 20.8 $ 53.0 $ 36.6
Share based compensation recognition period 4 years    
Ordinary Shares [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Unexercised options and shares, percent 2.20%    
Top of range [member] | Ordinary Shares [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Maximum number of ordinary shares that can be issued 41,076,635    
Maximum number of shares that can be issued to an individual 4,107,663