Shareholders' equity (Details) $ / shares in Units, $ in Millions	Jun. 30, 2018 SFr / shares shares	May 31, 2018 $ / shares	Dec. 31, 2017 SFr / shares shares	Dec. 21, 2017 USD ($)	May 31, 2017 $ / shares	Nov. 30, 2016 USD ($)	May 31, 2016 $ / shares
Equity, Class of Treasury Stock [Line Items]
Common Shares, par value | SFr / shares	SFr 24.15		SFr 24.15
Annual dividend per share approved by shareholders | $ / shares		$ 2.92			$ 2.84		$ 2.76
Common Shares in treasury, shares | shares	16,281,700		15,950,685
Common Stock, Dividend Rate Approved | $ / shares		$ 0.73			$ 0.71		$ 0.69
Dec 2017 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount of share repurchase program | $				$ 1,000
Nov 2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount of share repurchase program | $						$ 1,000