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Commitments and contingencies - Narrative (Details) - CHF (SFr)
SFr in Millions
Dec. 31, 2023
Jul. 07, 2023
Dec. 31, 2022
Disclosure of amounts to be recovered or settled after twelve months for classes of assets and liabilities that contain amounts to be recovered or settled both no more and more than twelve months after reporting date [line items]      
Gross lease liabilities SFr 385.7   SFr 282.9
Belgium      
Disclosure of amounts to be recovered or settled after twelve months for classes of assets and liabilities that contain amounts to be recovered or settled both no more and more than twelve months after reporting date [line items]      
Bank guarantee 27.9    
Gross lease liabilities 122.5   0.0
Atlanta      
Disclosure of amounts to be recovered or settled after twelve months for classes of assets and liabilities that contain amounts to be recovered or settled both no more and more than twelve months after reporting date [line items]      
Bank guarantee 111.9   126.1
Gross lease liabilities     254.9
Bank overdraft facility four, CHF      
Disclosure of amounts to be recovered or settled after twelve months for classes of assets and liabilities that contain amounts to be recovered or settled both no more and more than twelve months after reporting date [line items]      
Bank guarantee 155.3    
Credit facility, maximum borrowing capacity   SFr 700.0  
Group cash pool participants      
Disclosure of amounts to be recovered or settled after twelve months for classes of assets and liabilities that contain amounts to be recovered or settled both no more and more than twelve months after reporting date [line items]      
Gross bank balances before cash pooling offset 112.6   202.4
Contingent liability for guarantees      
Disclosure of amounts to be recovered or settled after twelve months for classes of assets and liabilities that contain amounts to be recovered or settled both no more and more than twelve months after reporting date [line items]      
Estimated financial effect of contingent liabilities 155.6   SFr 126.1
Leased locations      
Disclosure of amounts to be recovered or settled after twelve months for classes of assets and liabilities that contain amounts to be recovered or settled both no more and more than twelve months after reporting date [line items]      
Gross lease liabilities SFr 245.8