Share capital - Schedule of Shares Outstanding (Details) - shares	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Class A
Increase (decrease) in number of shares [Roll forward]
Number of shares authorized at beginning of period (in shares)	302,455,664	299,998,125
Number of shares held in treasury at start of period (in shares)	(13,159,321)	(15,782,848)
Number of shares outstanding at beginning of period (in shares)	289,296,343	284,215,277
Number of shares authorized at end of period (in shares)	306,847,124	302,455,664
Number of shares held in treasury at end of period (in shares)	(9,973,771)	(13,159,321)
Number of shares outstanding at end of period (in shares)	296,873,353	289,296,343
Class B
Increase (decrease) in number of shares [Roll forward]
Number of shares authorized at beginning of period (in shares)	345,437,500	345,437,500
Number of shares outstanding at beginning of period (in shares)	345,437,500	345,437,500
Number of shares authorized at end of period (in shares)	341,241,680	345,437,500
Number of shares outstanding at end of period (in shares)	341,241,680	345,437,500
Authorized registered shares \| Class A
Increase (decrease) in number of shares [Roll forward]
Capital increase from conditional capital (in shares)	3,339,378	2,457,539
Capital increase from capital band (in shares)
Conversion of Class B shares to Class A shares (in shares)	1,052,082
Sale of shares held by On in treasury (in shares)		0
Purchase of On shares from employees (sell-to-cover) at current market price (in shares)	0	0
Authorized registered shares \| Class B
Increase (decrease) in number of shares [Roll forward]
Capital increase from conditional capital (in shares)		0
Capital increase from capital band (in shares)	6,325,000
Conversion of Class B shares to Class A shares (in shares)	(10,520,820)
Sale of shares held by On in treasury (in shares)	0	0
Purchase of On shares from employees (sell-to-cover) at current market price (in shares)	0	0
Shares held by On in treasury \| Class A
Increase (decrease) in number of shares [Roll forward]
Capital increase from conditional capital (in shares)		0
Capital increase from capital band (in shares)
Conversion of Class B shares to Class A shares (in shares)
Sale of shares held by On in treasury (in shares)	3,189,789	2,633,847
Purchase of On shares from employees (sell-to-cover) at current market price (in shares)	(4,239)	(10,320)
Outstanding shares \| Class A
Increase (decrease) in number of shares [Roll forward]
Capital increase from conditional capital (in shares)	3,339,378	2,457,539
Capital increase from capital band (in shares)
Conversion of Class B shares to Class A shares (in shares)	1,052,082
Sale of shares held by On in treasury (in shares)	3,189,789	2,633,847
Purchase of On shares from employees (sell-to-cover) at current market price (in shares)	(4,239)	(10,320)
Outstanding shares \| Class B
Increase (decrease) in number of shares [Roll forward]
Capital increase from conditional capital (in shares)		0
Capital increase from capital band (in shares)	6,325,000
Conversion of Class B shares to Class A shares (in shares)	(10,520,820)
Sale of shares held by On in treasury (in shares)	0	0
Purchase of On shares from employees (sell-to-cover) at current market price (in shares)	0	0