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Shareholders' equity Schedule of Dividends Declared (Details) | 12 Months Ended
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Dec. 31, 2017 $ / shares | Dec. 31, 2017 SFr / shares | Dec. 31, 2016 $ / shares | Dec. 31, 2016 SFr / shares | Dec. 31, 2015 $ / shares | Dec. 31, 2015 SFr / shares
Switzerland, Francs |  |  |  |  |  |
Dividends Declared [Line Items] |  |  |  |  |  |
Dividends - par value reduction | SFr / shares |  | SFr 0 |  | SFr 0 |  | SFr 0.62
Dividends - distributed from capital contribution reserves | SFr / shares |  | 2.76 |  | 2.70 |  | 1.94
Total dividend distributions per common share | SFr / shares |  | SFr 2.76 |  | SFr 2.70 |  | SFr 2.56
United States of America, Dollars |  |  |  |  |  |
Dividends Declared [Line Items] |  |  |  |  |  |
Dividends - par value reduction | $ / shares | $ 0 |  | $ 0 |  | $ 0.65 |
Dividends - distributed from capital contribution reserves | $ / shares | 2.82 |  | 2.74 |  | 2.01 |
Total dividend distributions per common share | $ / shares | $ 2.82 |  | $ 2.74 |  | $ 2.66 |

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- Definition [Line Items] for Dividends Declared [Table] + References No definition available. + Details
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- Definition Aggregate dividends declared during the period for each share of common stock outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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