Shareholders' equity Dividends Declared (Details)	3 Months Ended				6 Months Ended
	Jun. 30, 2018 $ / shares	Jun. 30, 2018 SFr / shares	Jun. 30, 2017 $ / shares	Jun. 30, 2017 SFr / shares	Jun. 30, 2018 $ / shares	Jun. 30, 2018 SFr / shares	Jun. 30, 2017 $ / shares	Jun. 30, 2017 SFr / shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Common Stock, Dividends, Per Share, Declared | (per share)	$ 0.73	SFr 0.73	$ 0.71	SFr 0.69	$ 1.44	SFr 1.39	$ 1.40	SFr 1.38