Shareholders' equity (Details) - SFr / shares	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Stockholders' Equity Note [Abstract]
Common Shares, par value	SFr 24.15	SFr 24.15
Common Shares in treasury, shares	27,928,305	26,872,639
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	2,054,334