Shareholders' equity Shareholders' equity (Details) $ / shares in Units, $ in Billions	Sep. 30, 2017 SFr / shares shares	May 31, 2017 $ / shares	Dec. 31, 2016 SFr / shares shares	Nov. 30, 2016 USD ($)	May 31, 2016 $ / shares	May 31, 2015 $ / shares
Equity, Class of Treasury Stock [Line Items]
Common Shares, par value | SFr / shares	SFr 24.15		SFr 24.15
Common Shares in treasury, shares | shares	15,625,345		13,815,148
Common Stock, Dividend Rate Approved		$ 0.71			$ 0.69	$ 0.67
Annual dividend per share approved by shareholders		$ 2.84			$ 2.76	$ 2.68
Nov 2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount of share repurchase program | $				$ 1