Shareholders' equity Schedule of Dividends Declared (Details)	12 Months Ended
	Dec. 31, 2017 $ / shares	Dec. 31, 2017 SFr / shares	Dec. 31, 2016 $ / shares	Dec. 31, 2016 SFr / shares	Dec. 31, 2015 $ / shares	Dec. 31, 2015 SFr / shares
Switzerland, Francs
Dividends Declared [Line Items]
Dividends - par value reduction | SFr / shares		SFr 0		SFr 0		SFr 0.62
Dividends - distributed from capital contribution reserves | SFr / shares		2.76		2.70		1.94
Total dividend distributions per common share | SFr / shares		SFr 2.76		SFr 2.70		SFr 2.56
United States of America, Dollars
Dividends Declared [Line Items]
Dividends - par value reduction | $ / shares	$ 0		$ 0		$ 0.65
Dividends - distributed from capital contribution reserves | $ / shares	2.82		2.74		2.01
Total dividend distributions per common share | $ / shares	$ 2.82		$ 2.74		$ 2.66