Shareholders' equity Shareholders' equity (Details) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017 $ / shares SFr / shares shares	Jun. 30, 2017 SFr / shares shares	Jun. 30, 2016 $ / shares	Jun. 30, 2016 SFr / shares	Jun. 30, 2017 $ / shares SFr / shares shares	Jun. 30, 2017 SFr / shares shares	Jun. 30, 2016 $ / shares	Jun. 30, 2016 SFr / 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			SFr 24.15	SFr 24.15				SFr 24.15
Common Stock, Dividend Rate Declared | (per share)	SFr 0.71	SFr 0.69	$ 0.69	SFr 0.68	SFr 1.40	SFr 1.38	$ 1.36	SFr 1.34
Common Shares in treasury, shares | shares	14,408,723	14,408,723			14,408,723	14,408,723				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 | $											$ 1,000