Pensions and other post-retirement benefits - Summary of mortality assumptions (Details)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Life expectancy at age 60 for a male currently aged 60 | United Kingdom
Disclosure of defined benefit plans [line items]
Mortality assumptions	26.9	27.3	27.4
Life expectancy at age 60 for a male currently aged 60 | US
Disclosure of defined benefit plans [line items]
Mortality assumptions	24.7	24.9	25.1
Life expectancy at age 60 for a male currently aged 60 | Eurozone
Disclosure of defined benefit plans [line items]
Mortality assumptions	25.7	25.7	25.6
Life expectancy at age 60 for a male currently aged 40 | United Kingdom
Disclosure of defined benefit plans [line items]
Mortality assumptions	28.4	28.9	28.9
Life expectancy at age 60 for a male currently aged 40 | US
Disclosure of defined benefit plans [line items]
Mortality assumptions	26.4	26.7	26.9
Life expectancy at age 60 for a male currently aged 40 | Eurozone
Disclosure of defined benefit plans [line items]
Mortality assumptions	28.2	28.3	28.1
Life expectancy at age 60 for a female currently aged 60 | United Kingdom
Disclosure of defined benefit plans [line items]
Mortality assumptions	28.8	28.7	28.8
Life expectancy at age 60 for a female currently aged 60 | US
Disclosure of defined benefit plans [line items]
Mortality assumptions	27.7	28.0	28.5
Life expectancy at age 60 for a female currently aged 60 | Eurozone
Disclosure of defined benefit plans [line items]
Mortality assumptions	29.0	29.1	29.0
Life expectancy at age 60 for a female currently aged 40 | United Kingdom
Disclosure of defined benefit plans [line items]
Mortality assumptions	30.4	30.5	30.6
Life expectancy at age 60 for a female currently aged 40 | US
Disclosure of defined benefit plans [line items]
Mortality assumptions	29.2	29.7	30.1
Life expectancy at age 60 for a female currently aged 40 | Eurozone
Disclosure of defined benefit plans [line items]
Mortality assumptions	31.2	31.2	31.2