Shareholders' equity Shareholders' equity (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) SFr / shares shares	Mar. 31, 2015 USD ($) $ / shares shares	Mar. 31, 2015 USD ($) SFr / shares	Mar. 31, 2016 SFr / shares	Dec. 31, 2015 SFr / shares shares	May. 31, 2015 $ / shares	Jan. 01, 2015 USD ($)	May. 31, 2014 $ / shares
Equity, Class of Treasury Stock [Line Items]
Common Shares in treasury, shares | shares	15,500,344	15,500,344				18,268,971
Par Value Reduction | (per share)	$ 0	SFr 0	$ 0.65	SFr 0.62
Dividends Declared From Additional Paid In Capital | (per share)	0.67	0.66	0.00	0
Annual dividend per share approved by shareholders | $ / shares							$ 2.68		$ 2.60
Dividends declared per common share | (per share)	0.67	SFr 0.66	0.65	SFr 0.62
Common Stock, Dividend Rate, Per-Dollar-Amount | $ / shares	$ 0.67		$ 0.65
Common Shares, par value | SFr / shares					SFr 24.15	SFr 24.15
2015 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Common Shares repurchased	$ 0		$ 340
Repurchase of outstanding common shares, shares | shares	0		3,027,463
Share repurchase authorization remains	$ 0	SFr 0	$ 1,160	SFr 1,160
Stock repurchase program, authorized amount								$ 1,500