Income taxes - Narrative (Details) - CHF (SFr) SFr in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Income Taxes [Abstract]
Income tax benefit / (expense)	SFr (20.7)	SFr 6.4	SFr (33.5)	SFr (0.8)
Effective tax rate	16.50%	13.50%	13.90%	5.10%

- References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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+ Details