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, 2016
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 2014
Dividends Payable [Line Items]        
Dividend, Total Amount   $ 246.5 $ 390.1 $ 199.1
Dividend, Record Date Jun. 09, 2016      
Dividend, Payment Date Aug. 05, 2016      
125 Year Anniversary Special Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.28 $ 0.28 $ 0.28
Dividend, Total Amount   $ 124.6 $ 124.6 $ 124.6
Dividend, Announcement Date   Feb. 28, 2014 Feb. 28, 2014 Feb. 28, 2014
Dividend, Record Date   Mar. 21, 2014 Mar. 21, 2014 Mar. 21, 2014
Dividend, Payment Date   May 30, 2014 May 30, 2014 May 30, 2014
FY 2015 First Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.08 $ 0.08 $ 0.08
Dividend, Total Amount   $ 34.2 $ 34.2 $ 34.2
Dividend, Announcement Date   Nov. 19, 2014 Nov. 19, 2014 Nov. 19, 2014
Dividend, Record Date   Dec. 23, 2014 Dec. 23, 2014 Dec. 23, 2014
Dividend, Payment Date   Feb. 27, 2015 Feb. 27, 2015 Feb. 27, 2015
FY 2014 First Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.08 $ 0.08 $ 0.08
Dividend, Total Amount   $ 35.5 $ 35.5 $ 35.5
Dividend, Announcement Date   Nov. 14, 2013 Nov. 14, 2013 Nov. 14, 2013
Dividend, Record Date   Dec. 19, 2013 Dec. 19, 2013 Dec. 19, 2013
Dividend, Payment Date   Mar. 28, 2014 Mar. 28, 2014 Mar. 28, 2014
FY 2014 Special Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.20 $ 0.20 $ 0.20
Dividend, Total Amount   $ 89.0 $ 89.0 $ 89.0
Dividend, Announcement Date   May 22, 2014 May 22, 2014 May 22, 2014
Dividend, Record Date   Jun. 12, 2014 Jun. 12, 2014 Jun. 12, 2014
Dividend, Payment Date   Aug. 08, 2014 Aug. 08, 2014 Aug. 08, 2014
FY 2014 Second Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.32 $ 0.32 $ 0.32
Dividend, Total Amount   $ 142.3 $ 142.3 $ 142.3
Dividend, Announcement Date   May 22, 2014 May 22, 2014 May 22, 2014
Dividend, Record Date   Jun. 12, 2014 Jun. 12, 2014 Jun. 12, 2014
Dividend, Payment Date   Aug. 08, 2014 Aug. 08, 2014 Aug. 08, 2014
FY 2013 Special Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.24 $ 0.24 $ 0.24
Dividend, Total Amount   $ 106.1 $ 106.1 $ 106.1
Dividend, Announcement Date   May 23, 2013 May 23, 2013 May 23, 2013
Dividend, Record Date   Jun. 28, 2013 Jun. 28, 2013 Jun. 28, 2013
Dividend, Payment Date   Jul. 26, 2013 Jul. 26, 2013 Jul. 26, 2013
FY 2012 Second Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.13 $ 0.13 $ 0.13
Dividend, Total Amount   $ 57.5 $ 57.5 $ 57.5
Dividend, Announcement Date   May 23, 2013 May 23, 2013 May 23, 2013
Dividend, Record Date   Jun. 28, 2013 Jun. 28, 2013 Jun. 28, 2013
Dividend, Payment Date   Jul. 26, 2013 Jul. 26, 2013 Jul. 26, 2013
FY 2016 First Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.09 $ 0.09 $ 0.09
Dividend, Total Amount   $ 39.7 $ 39.7 $ 39.7
Dividend, Announcement Date   Nov. 19, 2015 Nov. 19, 2015 Nov. 19, 2015
Dividend, Record Date   Dec. 23, 2015 Dec. 23, 2015 Dec. 23, 2015
Dividend, Payment Date   Feb. 26, 2016 Feb. 26, 2016 Feb. 26, 2016
FY 2015 Special Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.22 $ 0.22 $ 0.22
Dividend, Total Amount   $ 92.8 $ 92.8 $ 92.8
Dividend, Announcement Date   May 21, 2015 May 21, 2015 May 21, 2015
Dividend, Record Date   Jun. 11, 2015 Jun. 11, 2015 Jun. 11, 2015
Dividend, Payment Date   Aug. 07, 2015 Aug. 07, 2015 Aug. 07, 2015
FY 2015 Second Half Dividend [Member]        
Dividends Payable [Line Items]        
Interim ordinary dividend to shareholders   $ 0.27 $ 0.27 $ 0.27
Dividend, Total Amount   $ 114.0 $ 114.0 $ 114.0
Dividend, Announcement Date   May 21, 2015 May 21, 2015 May 21, 2015
Dividend, Record Date   Jun. 11, 2015 Jun. 11, 2015 Jun. 11, 2015
Dividend, Payment Date   Aug. 07, 2015 Aug. 07, 2015 Aug. 07, 2015