Retirement benefits - Summary of Principal Financial Assumptions Used to Determine Benefit Obligations (Detail)	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
United Kingdom
Disclosure of defined benefit plan actuarial assumptions [line items]
Pension increases	3.00%	3.20%	3.10%
Inflation rate	3.00%	3.20%	3.10%
Discount rate:	5.60%	5.60%	4.80%
United States
Disclosure of defined benefit plan actuarial assumptions [line items]
Discount rate:	5.00%	5.30%	4.70%
Ultimate rate that the cost rate trends to	4.50%	4.50%	4.50%
United States \| Pre-65 (ultimate rate reached in 2036)
Disclosure of defined benefit plan actuarial assumptions [line items]
US healthcare cost trend rate assumed for the next year:	8.20%	8.60%	7.80%
United States \| Post-65 (ultimate rate reached in 2036)
Disclosure of defined benefit plan actuarial assumptions [line items]
US healthcare cost trend rate assumed for the next year:	9.50%	9.70%	8.60%
US post-retirement plan
Disclosure of defined benefit plan actuarial assumptions [line items]
Discount rate:	5.00%	5.30%	4.70%

The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [domain]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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The expected rate of inflation that is used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [domain]; Defined benefit obligation, at present value; Actuarial assumptions [domain]]

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The expected rate of increases in pension that is used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [domain]; Defined benefit obligation, at present value; Actuarial assumptions [domain]]

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Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [domain]]

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Disclosure Of Defined Benefit Plan Actuarial Assumptions [line items].

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Ultimate rate that the cost trend rate trends to.

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