Shareholders' equity Shareholders' equity (Details) $ / shares in Units, $ in Millions	1 Months Ended			3 Months Ended				9 Months Ended
	May 31, 2016 $ / shares	May 31, 2015 $ / shares	May 31, 2014 $ / shares	Sep. 30, 2016 $ / shares SFr / shares shares	Sep. 30, 2016 SFr / shares shares	Sep. 30, 2015 USD ($) $ / shares	Sep. 30, 2015 SFr / shares	Sep. 30, 2016 $ / shares SFr / shares shares	Sep. 30, 2016 SFr / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2015 SFr / shares	Jan. 01, 2016 USD ($)	Dec. 31, 2015 SFr / shares shares	Jan. 01, 2015 USD ($)
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 Approved by Shareholders | $ / shares	$ 0.69	$ 0.67	$ 0.65
Dividends declared per common share | (per share)				SFr 0.69	SFr 0.67	$ 0.67	SFr 0.65	SFr 2.05	SFr 2.01	$ 1.99	SFr 1.89
Common Shares in treasury, shares | shares				14,497,754	14,497,754			14,497,754	14,497,754				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 0.67	$ 0.67	SFr 0.65	SFr 2.05	SFr 2.01	$ 1.34	SFr 1.27
2015 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Common Shares repurchased						$ 0				$ 734
Repurchase of outstanding common shares, shares | shares										6,677,663
Authorized amount of share repurchase program														$ 1,500
2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount of share repurchase program												$ 0