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Share Capital - Additional Information (Detail)
 12 Months Ended
Mar. 30, 2018
USD ($)
shares
Mar. 16, 2016
USD ($)
shares
Mar. 31, 2018
USD ($)
shares
Mar. 31, 2018
GBP (£)
£ / shares
shares
Mar. 31, 2017
USD ($)
shares
Mar. 31, 2017
GBP (£)
£ / shares
shares
Mar. 31, 2016
USD ($)
shares
Mar. 31, 2018
$ / shares
shares
Mar. 16, 2018
£ / shares
Mar. 31, 2017
$ / shares
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       4,400,000   3,300,000 1,100,000 4,400,000   3,300,000
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       £ 0.10   £ 0.10        
Number of shares outstanding       50,434,080   50,012,559   50,434,080   50,012,559
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       £ 0.10   £ 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     1,100,000 1,100,000 2,200,000 2,200,000        
Shares repurchased value | $     $ 39,546,000   $ 64,224,000          
Shares repurchased transaction costs | $     $ 17,000   33,000          
Shares repurchased cancellation fees | $         $ 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