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, 2020
Mar. 31, 2019
Mar. 31, 2018
Dividends Payable [Line Items]      
Dividend, Total Amount $ 158.6 $ 172.1 $ 177.5
Withholding taxes on Dividends $ 2.7    
FY 2020 first half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.10    
Dividend, Total Amount $ 44.7    
Dividend, Announcement Date Nov. 07, 2019    
Dividend, Record Date Nov. 18, 2019    
Dividend, Payment Date Dec. 20, 2019    
FY 2019 second half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.26    
Dividend, Total Amount $ 113.9    
Dividend, Announcement Date May 21, 2019    
Dividend, Record Date Jun. 06, 2019    
Dividend, Payment Date Aug. 02, 2019    
FY 2019 first half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.10    
Dividend, Total Amount $ 43.6    
Dividend, Announcement Date Nov. 08, 2018    
Dividend, Record Date Dec. 12, 2018    
Dividend, Payment Date Feb. 22, 2019    
FY 2018 second half dividend      
Dividends Payable [Line Items]      
Interim ordinary dividend to shareholders (in dollars per share) $ 0.30    
Dividend, Total Amount $ 128.5    
Dividend, Announcement Date May 22, 2018    
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