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Provision for post-employment benefits - Sensitivity Analysis of Significant Actuarial Assumptions (Detail) - GBP (£) £ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Discount rate [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Assumption, basis point increase | 0.25% | 0.25%
Assumption, basis point decrease | 0.25% | 0.25%
Discount rate [member] | United Kingdom [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | £ (8.2) | £ (9.8)
Increase/(decrease) in benefit obligation, due to decrease in assumption | 8.5 | 10.3
Discount rate [member] | North America [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | (7.5) | (8.8)
Increase/(decrease) in benefit obligation, due to decrease in assumption | 7.7 | 9.1
Discount rate [member] | Western Continental Europe [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | (3.8) | (8.7)
Increase/(decrease) in benefit obligation, due to decrease in assumption | 3.9 | 9.3
Discount rate [member] | Asia Pacific, Latin America, Africa & Middle East and Central & Eastern Europe [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | (0.7) | (0.7)
Increase/(decrease) in benefit obligation, due to decrease in assumption | £ 0.7 | £ 0.7
Rate of increase in salaries [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Assumption, basis point increase | 0.25% | 0.25%
Assumption, basis point decrease | 0.25% | 0.25%
Rate of increase in salaries [member] | Western Continental Europe [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | £ 0.8 | £ 1.3
Increase/(decrease) in benefit obligation, due to decrease in assumption | (0.8) | (1.2)
Rate of increase in salaries [member] | Asia Pacific, Latin America, Africa & Middle East and Central & Eastern Europe [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | 0.6 | 0.7
Increase/(decrease) in benefit obligation, due to decrease in assumption | £ (0.6) | £ (0.6)
Rate of increase in pensions in payment [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Assumption, basis point increase | 0.25% | 0.25%
Assumption, basis point decrease | 0.25% | 0.25%
Rate of increase in pensions in payment [member] | United Kingdom [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | £ 0.7 | £ 1.3
Increase/(decrease) in benefit obligation, due to decrease in assumption | (0.6) | (0.8)
Rate of increase in pensions in payment [member] | Western Continental Europe [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | 1.9 | 5.3
Increase/(decrease) in benefit obligation, due to decrease in assumption | £ (1.9) | £ (5.0)
Life expectancy [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Assumption, increase in longevity | 1 year | 1 year
Life expectancy [member] | United Kingdom [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | £ 11.7 | £ 13.6
Life expectancy [member] | North America [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | 5.9 | 5.7
Life expectancy [member] | Western Continental Europe [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase/(decrease) in benefit obligation, due to increase in assumption | £ 4.3 | £ 6.9

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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by a decrease in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2019-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_145_a&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2019-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_145_a&doctype=Standard -URIDate 2019-03-27 + Details
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