Shareholders' equity Shareholders' equity (Details) $ / shares in Units, $ in Millions	3 Months Ended					6 Months Ended
	Jun. 30, 2016 $ / shares SFr / shares shares	Jun. 30, 2016 SFr / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2015 SFr / shares	Mar. 31, 2015 $ / shares	Jun. 30, 2016 $ / shares SFr / shares shares	Jun. 30, 2016 SFr / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2015 SFr / shares	May 31, 2016 $ / 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, par value | SFr / shares	SFr 24.15	SFr 24.15				SFr 24.15	SFr 24.15				SFr 24.15
Common Stock, Dividend Rate, Per-Dollar-Amount | $ / shares			$ 0.67		$ 0.65
Dividends declared per common share | (per share)	SFr 0.69	SFr 0.68	0.67	SFr 0.62		SFr 1.36	SFr 1.34	$ 1.32	SFr 1.24
Common Shares in treasury, shares | shares	14,770,884	14,770,884				14,770,884	14,770,884				18,268,971
Annual dividend per share approved by shareholders | $ / shares										$ 2.76		$ 2.68		$ 2.60
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Dividends declared per common share | (per share)	SFr 0	SFr 0	0	0		SFr 0	SFr 0	0.65	0.62
Additional Paid-in Capital [Member]
Equity, Class of Treasury Stock [Line Items]
Dividends declared per common share | (per share)	SFr 0.69	SFr 0.68	$ 0.67	SFr 0.62		SFr 1.36	SFr 1.34	$ 0.67	SFr 0.62
2015 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Common Shares repurchased | $			$ 394					$ 734
Repurchase of outstanding common shares, shares | shares			3,650,200					6,677,663
Authorized amount of share repurchase program | $													$ 1,500