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]]

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The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

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The estimate of the expected contributions to be made to a defined benefit plan for the next annual reporting period. [Refer: Defined benefit plans [member]]

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Provision for post-employment benefits - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure of defined benefit plans [line items]
Employer contributions (for funded plans) and benefit payments (for unfunded plans)	£ 37.1	£ 44.9	£ 68.2
Employer contributions (for funded plans) and benefit payments (for unfunded plans), expected in 2020.	£ 25.0
Years life expectancy after age 65 - Current pensioners (at age 65) - male [member]
Disclosure of defined benefit plans [line items]
Actuarial assumption of life expectancy	22 years 2 months 12 days	22 years 2 months 12 days
Years life expectancy after age 65 - Current pensioners (at age 65) - female [member]
Disclosure of defined benefit plans [line items]
Actuarial assumption of life expectancy	23 years 8 months 12 days	23 years 10 months 24 days
Years life expectancy after age 65 - Future pensioners (current age 45) - male [member]
Disclosure of defined benefit plans [line items]
Actuarial assumption of life expectancy	23 years 9 months 18 days	24 years
Years life expectancy after age 65 - Future pensioners (current age 45) - female [member]
Disclosure of defined benefit plans [line items]
Actuarial assumption of life expectancy	25 years 4 months 24 days	25 years 8 months 12 days