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Capital Management and Dividends - Summary of Dividends Declared (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified |
12 Months Ended | ||
|---|---|---|---|
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Mar. 31, 2014
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Mar. 31, 2013
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Mar. 31, 2012
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| Dividends Payable [Line Items] | |||
| Dividend, Total Amount | $ 199.1 | $ 188.5 | $ 17.4 |
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125 Year Anniversary Special Dividend [Member]
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| Dividends Payable [Line Items] | |||
| Interim ordinary dividend to shareholders | $ 0.28 | $ 0.28 | |
| Dividend, Total Amount | 124.6 | 124.6 | |
| Dividend, Announcement Date | Feb. 28, 2014 | Feb. 28, 2014 | |
| Dividend, Record Date | Mar. 21, 2014 | Mar. 21, 2014 | |
| Dividend, Payment Date | May 30, 2014 | May 30, 2014 | |
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FY 2014 First Half Dividend [Member]
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| Dividends Payable [Line Items] | |||
| Interim ordinary dividend to shareholders | $ 0.08 | $ 0.08 | |
| Dividend, Total Amount | 35.5 | 35.5 | |
| Dividend, Announcement Date | Nov. 14, 2013 | Nov. 14, 2013 | |
| Dividend, Record Date | Dec. 19, 2013 | Dec. 19, 2013 | |
| Dividend, Payment Date | Mar. 28, 2014 | Mar. 28, 2014 | |
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FY 2013 Special Dividend [Member]
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| Dividends Payable [Line Items] | |||
| Interim ordinary dividend to shareholders | $ 0.24 | $ 0.24 | |
| Dividend, Total Amount | 106.1 | 106.1 | |
| Dividend, Announcement Date | May 23, 2013 | May 23, 2013 | |
| Dividend, Record Date | Jun. 28, 2013 | Jun. 28, 2013 | |
| Dividend, Payment Date | Jul. 26, 2013 | Jul. 26, 2013 | |
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FY 2013 Second Half Dividend [Member]
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| Dividends Payable [Line Items] | |||
| Interim ordinary dividend to shareholders | $ 0.13 | $ 0.13 | |
| Dividend, Total Amount | 57.5 | 57.5 | |
| Dividend, Announcement Date | May 23, 2013 | May 23, 2013 | |
| Dividend, Record Date | Jun. 28, 2013 | Jun. 28, 2013 | |
| Dividend, Payment Date | Jul. 26, 2013 | Jul. 26, 2013 | |
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FY 2013 First Half Dividend [Member]
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| Dividends Payable [Line Items] | |||
| Interim ordinary dividend to shareholders | $ 0.05 | $ 0.05 | |
| Dividend, Total Amount | 22.1 | 22.1 | |
| Dividend, Announcement Date | Nov. 15, 2012 | Nov. 15, 2012 | |
| Dividend, Record Date | Dec. 18, 2012 | Dec. 18, 2012 | |
| Dividend, Payment Date | Jan. 25, 2013 | Jan. 25, 2013 | |
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FY 2012 Second Half Dividend [Member]
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| Dividends Payable [Line Items] | |||
| Interim ordinary dividend to shareholders | $ 0.38 | $ 0.38 | |
| Dividend, Total Amount | $ 166.4 | $ 166.4 | |
| Dividend, Announcement Date | May 21, 2012 | May 21, 2012 | |
| Dividend, Record Date | Jun. 29, 2012 | Jun. 29, 2012 | |
| Dividend, Payment Date | Jul. 23, 2012 | Jul. 23, 2012 | |