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

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

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Capital Management and Dividends - Summary of Dividends Declared or Paid (Detail) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2015
Dividends Payable [Line Items]        
Dividend, Total Amount   $ 176.8 $ 246.5 $ 390.1
Dividend, Record Date Jun. 08, 2017      
Dividend, Payment Date Aug. 04, 2017      
FY 2017 First Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 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 [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 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 [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 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 [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 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 [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 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    
FY 2015 First Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.08    
Dividend, Total Amount   $ 34.2    
Dividend, Announcement Date   Nov. 19, 2014    
Dividend, Record Date   Dec. 23, 2014    
Dividend, Payment Date   Feb. 27, 2015    
FY 2014 Special Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.20    
Dividend, Total Amount   $ 89.0    
Dividend, Announcement Date   May 22, 2014    
Dividend, Record Date   Jun. 12, 2014    
Dividend, Payment Date   Aug. 08, 2014    
FY 2014 Second Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.32    
Dividend, Total Amount   $ 142.3    
Dividend, Announcement Date   May 22, 2014    
Dividend, Record Date   Jun. 12, 2014    
Dividend, Payment Date   Aug. 08, 2014    
125 Year Anniversary Special Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.28    
Dividend, Total Amount   $ 124.6    
Dividend, Announcement Date   Feb. 28, 2014    
Dividend, Record Date   Mar. 21, 2014    
Dividend, Payment Date   May 30, 2014