Shareholders' equity (Details)	6 Months Ended
	Jun. 30, 2021 SFr / shares shares	May 20, 2021 $ / shares	Dec. 31, 2020 SFr / shares shares	May 20, 2020 $ / shares	May 31, 2019 $ / shares
Stockholders' Equity Note [Abstract]
Common Shares, par value | SFr / shares	SFr 24.15		SFr 24.15
Common Shares in treasury, shares | shares	38,888,051		26,872,639
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures | shares	2,449,988
Common Stock, Dividend Rate Approved | $ / shares		$ 0.80		$ 0.78	$ 0.75
Annual dividend per share approved by shareholders | $ / shares		$ 3.20		$ 3.12	$ 3.00