Shareholders' equity Shareholders' equity (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017 $ / shares SFr / shares shares	Mar. 31, 2017 SFr / shares shares	Mar. 31, 2016 $ / shares	Mar. 31, 2016 SFr / 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
Common Stock, Dividend Rate Declared | (per share)	SFr 0.69	SFr 0.69	$ 0.67	SFr 0.66
Common Shares in treasury, shares | shares	12,560,845	12,560,845			13,815,148
Annual dividend per share approved by shareholders							$ 2.76	$ 2.68
Common Stock, Dividend Rate Approved							$ 0.69	$ 0.67
Nov 2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount of share repurchase program | $						$ 1,000