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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 |  |  |  |  |  |

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