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Goodwill and intangible assets - Narrative (Details) - business_group
 6 Months Ended 12 Months Ended
Dec. 31, 2022
Dec. 31, 2023
Disclosure of detailed information about intangible assets [line items]    
Number of business groups 5  
Cash flow projection period   5 years
Group's strategic plan, period   3 years
Bottom of range    
Disclosure of detailed information about intangible assets [line items]    
Pre-tax discount rate 7.40% 8.40%
Top of range    
Disclosure of detailed information about intangible assets [line items]    
Amortisation period of finite-life intangible assets   ten
Pre-tax discount rate 11.80% 20.00%