v3.26.1
Pensions and other post-retirement benefits - Disclosure of actuarial assumptions (Details)
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
UK | Pensions      
Disclosure of defined benefit plans [line items]      
Discount rate – past service 6.00% 5.73% 4.87%
Discount rate – future service 6.35% 5.95% 4.92%
Rate of increase in RPI – past service 3.17% 2.99% 3.05%
Rate of increase in RPI – future service 3.06% 2.85% 2.92%
Salary increases 3.32% 3.08% 3.10%
Assumed life expectations for a retiree age 65 today: Males 21 years 10 months 24 days 21 years 6 months 21 years 6 months
Assumed life expectations for a retiree age 65 today: Females 24 years 2 months 12 days 23 years 10 months 24 days 23 years 6 months
Assumed life expectations for a retiree age 65 today: In 20 years: Males 22 years 10 months 24 days 22 years 4 months 24 days 22 years 7 months 6 days
Assumed life expectations for a retiree age 65 today: In 20 years: Females 25 years 7 months 6 days 25 years 3 months 18 days 24 years 10 months 24 days
UK | Pensions | Active members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 10.00% 11.00%  
UK | Pensions | Deferred members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 7.00% 7.00%  
UK | Pensions | Pensioner members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 83.00% 82.00%  
US | Pensions      
Disclosure of defined benefit plans [line items]      
Discount rate – past service 5.60% 5.50% 5.15%
Discount rate – future service 5.60% 5.50% 5.15%
Salary increases 4.50% 4.50% 4.50%
Assumed life expectations for a retiree age 65 today: Males 21 years 10 months 24 days 21 years 9 months 18 days 21 years 7 months 6 days
Assumed life expectations for a retiree age 65 today: Females 23 years 10 months 24 days 23 years 9 months 18 days 23 years 10 months 24 days
Assumed life expectations for a retiree age 65 today: In 20 years: Males 23 years 6 months 23 years 4 months 24 days 23 years 3 months 18 days
Assumed life expectations for a retiree age 65 today: In 20 years: Females 25 years 6 months 25 years 4 months 24 days 25 years 6 months
US | Pensions | Active members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 42.00% 40.00%  
US | Pensions | Deferred members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 10.00% 10.00%  
US | Pensions | Pensioner members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 48.00% 50.00%  
US | Other post-retirement benefits      
Disclosure of defined benefit plans [line items]      
Discount rate – past service 5.60% 5.50% 5.15%
Discount rate – future service 5.60% 5.50% 5.15%
Salary increases 4.50% 4.50% 4.50%
Initial healthcare cost trend rate 7.10% 7.80% 7.10%
Ultimate healthcare cost trend rate 4.50% 4.50% 4.50%
US | Other post-retirement benefits | Active members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 30.00% 28.00%  
US | Other post-retirement benefits | Pensioner members      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation, member concentration percentage 70.00% 72.00%