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Capital Management and Dividends - Summary of Dividends Paid (Detail) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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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 |  |

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