Shareholders' equity (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Apr. 30, 2018 $ / shares	Dec. 31, 2017 USD ($) shares	Apr. 30, 2017 $ / shares	Dec. 31, 2016 USD ($) shares	Apr. 30, 2016 $ / shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2017 SFr / shares	May 31, 2017 $ / shares	Dec. 31, 2016 SFr / shares	Nov. 30, 2016 USD ($)	May 31, 2016 $ / shares	May 31, 2015 $ / shares	Nov. 30, 2014 USD ($)
Stockholders' Equity Note [Abstract]
Dividend installments | $ / shares	$ 0.71		$ 0.69		$ 0.67
The number of votes associated with one Common Share		1
The maximum ownership percentage for voting allowed for any one shareholder		10.00%
Annual dividend per share approved by shareholders | $ / shares								$ 2.84			$ 2.76	$ 2.68
Authorized Share Capital [Line Items]
Stock repurchase program authorized amount | $		$ 1,000								$ 1,000			$ 1,500
Common Shares, par value | SFr / shares							SFr 24.15		SFr 24.15
General Purpose
Authorized Share Capital [Line Items]
Authorized share capital for future issuance		200,000,000
Issuance of Debt
Authorized Share Capital [Line Items]
Authorized share capital for future issuance		33,000,000
Employee Benefit Plans
Authorized Share Capital [Line Items]
Authorized share capital for future issuance		25,410,929
Nov 2016 Stock Repurchase Plan [Member]
Authorized Share Capital [Line Items]
Treasury Stock, Shares, Acquired		5,866,612		0		6,677,663
Treasury Stock, Value, Acquired, Cost Method | $		$ 830		$ 0		$ 734