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Shareholders' equity Shareholders' equity (Details) $ / shares in Units, $ in Millions | 3 Months Ended |  |  |  |
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Mar. 31, 2017 $ / shares SFr / shares shares | Mar. 31, 2017 SFr / shares shares | Mar. 31, 2016 $ / shares | Mar. 31, 2016 SFr / shares | Dec. 31, 2016 SFr / shares shares | Nov. 30, 2016 USD ($) | May 31, 2016 $ / shares | May 31, 2015 $ / shares
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |
Common Shares, par value | SFr / shares | SFr 24.15 | SFr 24.15 |  |  | SFr 24.15 |  |  |
Common Stock, Dividend Rate Declared | (per share) | SFr 0.69 | SFr 0.69 | $ 0.67 | SFr 0.66 |  |  |  |
Common Shares in treasury, shares | shares | 12,560,845 | 12,560,845 |  |  | 13,815,148 |  |  |
Annual dividend per share approved by shareholders |  |  |  |  |  |  | $ 2.76 | $ 2.68
Common Stock, Dividend Rate Approved |  |  |  |  |  |  | $ 0.69 | $ 0.67
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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