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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 |  |  |  |  |  |  |

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