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Shareholders' equity (Detail) $ / shares in Units, $ in Millions | 12 Months Ended |  |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2020 SFr / shares shares | Dec. 31, 2019 SFr / shares shares | Dec. 31, 2018 shares | Feb. 01, 2021 USD ($) | Nov. 19, 2020 USD ($) | May 31, 2020 $ / shares | May 20, 2020 $ / shares | Nov. 21, 2019 USD ($) | May 31, 2019 $ / shares | Dec. 01, 2018 USD ($) | May 31, 2018 $ / shares | Dec. 31, 2017 shares | Dec. 21, 2017 USD ($)
Stockholders' Equity Note [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividend Rate Approved | $ / shares |  |  |  |  |  |  | $ 0.78 |  | $ 0.75 |  | $ 0.73 |  |
The number of votes associated with one Common Share | one |  |  |  |  |  |  |  |  |  |  |  |
Annual dividend per share approved by shareholders | $ / shares |  |  |  |  |  | $ 3.12 |  |  | $ 3.00 |  | $ 2.92 |  |
Authorized Share Capital [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Shares, par value | SFr / shares | SFr 24.15 | SFr 24.15 |  |  |  |  |  |  |  |  |  |  |
Common Shares, shares issued | shares | 477,605,264 | 479,783,864 | 479,783,864 |  |  |  |  |  |  |  |  | 479,783,864 |
Treasury Stock, Shares, Retired | shares | 2,178,600 | 0 | 0 |  |  |  |  |  |  |  |  |  |
The maximum ownership percentage for voting allowed for any one shareholder | 10.00% |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |
Nov 2019 Stock Repurchase Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Share Capital [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program authorized amount |  |  |  |  |  |  |  | $ 1,500 |  |  |  |  |
Nov 2020 Stock Repurchase Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Share Capital [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program authorized amount |  |  |  |  | $ 1,500 |  |  |  |  |  |  |  |
Subsequent Event [Member] | Nov 2020 Stock Repurchase Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Share Capital [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program authorized amount |  |  |  | $ 2,500 |  |  |  |  |  |  |  |  |
CB_Increase(Decrease)StockRepurchaseProgramAuthorizedAmount |  |  |  | $ 1,000 |  |  |  |  |  |  |  |  |

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