Shareholders' equity (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Apr. 30, 2019 $ / shares	Dec. 31, 2018 SFr / shares shares	Apr. 30, 2018 $ / shares	Apr. 30, 2017 $ / shares	Dec. 01, 2018 USD ($)	May 30, 2018 $ / shares	Dec. 31, 2017 SFr / shares	Dec. 21, 2017 USD ($)	May 31, 2017 $ / shares	Nov. 30, 2016 USD ($)	May 31, 2016 $ / shares
Stockholders' Equity Note [Abstract]
Dividend installments | $ / shares	$ 0.73		$ 0.71	$ 0.69
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						$ 2.92			$ 2.84		$ 2.76
Authorized Share Capital [Line Items]
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 | shares		200,000,000
Issuance of Debt
Authorized Share Capital [Line Items]
Authorized share capital for future issuance | shares		33,000,000
Employee Benefit Plans
Authorized Share Capital [Line Items]
Authorized share capital for future issuance | shares		25,410,929
Nov 2016 Stock Repurchase Plan [Member]
Authorized Share Capital [Line Items]
Stock repurchase program authorized amount | $										$ 1,000
2018 Stock Repurchase Plan [Member] [Domain]
Authorized Share Capital [Line Items]
Stock repurchase program authorized amount | $								$ 1,000
2019 Stock Repurchase Plan [Member]
Authorized Share Capital [Line Items]
Stock repurchase program authorized amount | $					$ 1,500