Shareholders' equity Dividends Declared (Details)	3 Months Ended				9 Months Ended
	Sep. 30, 2018 $ / shares	Sep. 30, 2018 SFr / shares	Sep. 30, 2017 $ / shares	Sep. 30, 2017 SFr / shares	Sep. 30, 2018 $ / shares	Sep. 30, 2018 SFr / shares	Sep. 30, 2017 $ / shares	Sep. 30, 2017 SFr / shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Common Stock, Dividends, Per Share, Declared | (per share)	$ 0.73	SFr 0.72	$ 0.71	SFr 0.68	$ 2.17	SFr 2.11	$ 2.11	SFr 2.06