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Shareholders' equity (Details) $ / shares in Units, $ in Millions | Sep. 30, 2018 SFr / shares shares | May 31, 2018 $ / shares | Dec. 31, 2017 SFr / shares shares | Dec. 21, 2017 USD ($) | May 31, 2017 $ / shares | Nov. 30, 2016 USD ($) | May 31, 2016 $ / shares
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Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Common Shares, par value | SFr / shares | SFr 24.15 |  | SFr 24.15 |  |  |  |
Annual dividend per share approved by shareholders | $ / shares |  | $ 2.92 |  |  | $ 2.84 |  | $ 2.76
Common Shares in treasury, shares | shares | 18,683,074 |  | 15,950,685 |  |  |  |
Common Stock, Dividend Rate Approved | $ / shares |  | $ 0.73 |  |  | $ 0.71 |  | $ 0.69
Dec 2017 Stock Repurchase Plan [Member] |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Authorized amount of share repurchase program | $ |  |  |  | $ 1,000 |  |  |
Nov 2016 Stock Repurchase Plan [Member] |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Authorized amount of share repurchase program | $ |  |  |  |  |  | $ 1,000 |

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