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Retirement benefits - Defined Benefit Plans - Life Expectancy Assumptions at Retirement Age (Detail) - Plan [member] | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
United Kingdom [member] | Current pensioners at 65 - male [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [1] | 24 years | 24 years | 24 years
United Kingdom [member] | Current pensioners at 65 - female [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [1] | 26 years | 26 years | 26 years
United Kingdom [member] | Future pensioners at 65 - male [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [2] | 25 years | 25 years | 25 years
United Kingdom [member] | Future pensioners at 65 - female [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [2] | 28 years | 28 years | 28 years
United States [member] | Current pensioners at 65 - male [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [1] | 22 years | 21 years | 21 years
United States [member] | Current pensioners at 65 - female [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [1] | 23 years | 23 years | 23 years
United States [member] | Future pensioners at 65 - male [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [2] | 23 years | 22 years | 22 years
United States [member] | Future pensioners at 65 - female [member] |  |  |  |
Disclosure Of Forecast Life Expectancies [line items] |  |  |  |
Actuarial assumption of life expectancy after retirement | [2] | 24 years | 24 years | 24 years
[1] | Relates to assumptions based on longevity (in years) following retirement at the end of the reporting period.
[2] | Relates to assumptions based on longevity (in years) relating to an employee retiring in [2040].

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- Definition The life expectancy after retirement used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 144 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_144&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition Disclosure Of Forecast Life Expectancies [line items] + References No definition available. + Details
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