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Share Capital - Additional Information (Detail)
 12 Months Ended
Mar. 30, 2018
USD ($)
shares
Mar. 16, 2016
USD ($)
shares
Mar. 31, 2019
USD ($)
shares
Mar. 31, 2019
GBP (£)
£ / shares
shares
Mar. 31, 2018
USD ($)
shares
Mar. 31, 2018
GBP (£)
£ / shares
shares
Mar. 31, 2017
USD ($)
shares
Mar. 31, 2016
USD ($)
shares
Mar. 31, 2019
$ / shares
shares
Mar. 31, 2018
$ / shares
shares
Mar. 16, 2018
£ / shares
Disclosure of classes of share capital [line items]                      
Authorized share capital shares       60,000,000   60,000,000     60,000,000 60,000,000  
Par value per share | (per share)       £ 0.10   £ 0.10     $ 0.16 $ 0.16  
Treasury shares [member]                      
Disclosure of classes of share capital [line items]                      
Number of shares outstanding       1,101,300   4,400,000 3,300,000 1,100,000 1,101,300 4,400,000  
Number of shares cancelled     (4,400,000) (4,400,000)              
Cancellation of treasury shares | $     $ 134,231,000                
Share capital [member]                      
Disclosure of classes of share capital [line items]                      
Number of shares outstanding       511,532,220   54,834,080 53,312,559 52,406,304 511,532,220 54,834,080  
Number of shares cancelled     (4,400,000) (4,400,000)              
Cancellation of treasury shares | $     $ (573,000)                
Share premium [member]                      
Disclosure of classes of share capital [line items]                      
Cancellation of treasury shares | $     $ (133,658,000)                
Ordinary shares [member]                      
Disclosure of classes of share capital [line items]                      
Authorized share capital amount | £       £ 6,100,000   £ 6,100,000          
Authorized share capital shares       60,000,000   60,000,000     60,000,000 60,000,000  
Par value per share | £ / shares       £ 0.1   £ 0.10          
Number of shares outstanding       50,051,920   50,434,080     50,051,920 50,434,080  
Preference shares [member]                      
Disclosure of classes of share capital [line items]                      
Authorized share capital shares       1,000,000   1,000,000     1,000,000 1,000,000  
Par value per share | £ / shares       £ 0.1   £ 0.10          
Number of shares outstanding       0   0     0 0  
American depositary shares [member]                      
Disclosure of classes of share capital [line items]                      
Par value per share | £ / shares                     £ 0.10
Number of ordinary shares represented by 1 American Depositary Shares   1                  
Number of shares repurchased     1,101,300 1,101,300 1,100,000 1,100,000 2,200,000        
Shares repurchased value | $     $ 56,362,000   $ 39,546,000   $ 64,224,000        
Shares repurchased transaction costs | $     11,000   $ 17,000   33,000        
Share repurchase cancellation fees | $     $ 55,000       $ 111,000 $ 55,000      
American depositary shares [member] | Top of range [member]                      
Disclosure of classes of share capital [line items]                      
Number of shares authorized to be repurchased 3,300,000 3,300,000                  
Price for repurchase of shares | $ $ 100 $ 50                  
Duration of share repurchase program 36 months                    
American depositary shares [member] | Bottom of range [member]                      
Disclosure of classes of share capital [line items]                      
Price for repurchase of shares | $ $ 10 $ 10                  
American depositary shares [member] | Treasury shares [member]                      
Disclosure of classes of share capital [line items]                      
Number of shares cancelled     4,400,000 4,400,000