Shareholders' equity (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) SFr / shares shares	Mar. 31, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) SFr / shares	Mar. 31, 2018 SFr / shares	Dec. 31, 2017 SFr / shares shares	Dec. 21, 2017 USD ($)	May 31, 2017 $ / shares	Nov. 30, 2016 USD ($)	May 31, 2016 $ / shares
Equity, Class of Treasury Stock [Line Items]
Common Shares, par value | SFr / shares					SFr 24.15	SFr 24.15
Annual dividend per share approved by shareholders | $ / shares								$ 2.84		$ 2.76
Common Shares in treasury, shares | shares	13,952,378	13,952,378				15,950,685
Common Stock, Dividend Rate Approved | $ / shares								$ 0.71		$ 0.69
Common Stock, Dividend Rate Declared | (per share)	$ 0.71	SFr 0.66	$ 0.69	SFr 0.69
Dec 2017 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount of share repurchase program							$ 1,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 1,000	SFr 1,000
Nov 2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount of share repurchase program									$ 1,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount			$ 860	SFr 860