Annual and transition report of foreign private issuers pursuant to Section 13 or 15(d)

Capital Management and Dividends - Summary of Dividends Paid (Detail)

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Capital Management and Dividends - Summary of Dividends Paid (Detail) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Dividends Payable [Line Items]      
Dividend, Total Amount $ 177.5 $ 176.8 $ 246.5
Dividend, Record Date Jun. 07, 2018    
Dividend, Payment Date Aug. 03, 2018    
FY 2018 first half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.10    
Dividend, Total Amount $ 46.2    
Dividend, Announcement Date Nov. 09, 2017    
Dividend, Record Date Dec. 13, 2017    
Dividend, Payment Date Feb. 23, 2018    
FY 2017 second half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.28    
Dividend, Total Amount $ 131.3    
Dividend, Announcement Date May 18, 2017    
Dividend, Record Date Jun. 08, 2017    
Dividend, Payment Date Aug. 04, 2017    
FY 2017 first half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.10    
Dividend, Total Amount $ 46.6    
Dividend, Announcement Date Nov. 17, 2016    
Dividend, Record Date Dec. 21, 2016    
Dividend, Payment Date Feb. 24, 2017    
FY 2016 second half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.29    
Dividend, Total Amount $ 130.2    
Dividend, Announcement Date May 19, 2016    
Dividend, Record Date Jun. 09, 2016    
Dividend, Payment Date Aug. 05, 2016    
FY 2016 first half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.09    
Dividend, Total Amount $ 39.7    
Dividend, Announcement Date Nov. 19, 2015    
Dividend, Record Date Dec. 23, 2015    
Dividend, Payment Date Feb. 26, 2016    
FY 2015 special dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.22    
Dividend, Total Amount $ 92.8    
Dividend, Announcement Date May 21, 2015    
Dividend, Record Date Jun. 11, 2015    
Dividend, Payment Date Aug. 07, 2015    
FY 2015 second half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.27    
Dividend, Total Amount $ 114.0    
Dividend, Announcement Date May 21, 2015    
Dividend, Record Date Jun. 11, 2015    
Dividend, Payment Date Aug. 07, 2015