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Retirement benefits - Defined Benefit Plans - Life Expectancy Assumptions at Retirement Age (Detail) - Plan [member] - yr | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
United Kingdom [member] |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |
Current pensioners at 65 - male | 24 | 24 | 24
Current pensioners at 65 - female | 26 | 26 | 26
Future pensioners at 65 - male | 25 | 25 | 25
Future pensioners at 65 - female | 28 | 28 | 28
United States [member] |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |
Current pensioners at 65 - male | 21 | 21 | 21
Current pensioners at 65 - female | 23 | 23 | 23
Future pensioners at 65 - male | 22 | 22 | 22
Future pensioners at 65 - female | 24 | 24 | 24

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