Shareholders' equity Dividends declared (Details)	3 Months Ended				9 Months Ended
	Sep. 30, 2017 $ / shares	Sep. 30, 2017 SFr / shares	Sep. 30, 2016 $ / shares	Sep. 30, 2016 SFr / shares	Sep. 30, 2017 $ / shares	Sep. 30, 2017 SFr / shares	Sep. 30, 2016 $ / shares	Sep. 30, 2016 SFr / shares
Dividends Declared [Abstract]
Common Stock, Dividend Rate Declared | (per share)	$ 0.71	SFr 0.68	$ 0.69	SFr 0.67	$ 2.11	SFr 2.06	$ 2.05	SFr 2.01