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Principles of consolidation (Details) | 12 Months Ended
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Dec. 31, 2017 € / shares pure / € | Dec. 31, 2016 € / shares pure / € | Dec. 31, 2015 € / shares pure / €
U.S. dollar [Member] |  |  |
DisclosureOfPrinciplesOfConsolidationLineItems [Line Items] |  |  |
Average foreign exchange rate | 1.13 | 1.11 | 1.11
Closing foreign exchange rate | 1.20 | 1.05 | 1.09
Pound Sterling [Member] |  |  |
DisclosureOfPrinciplesOfConsolidationLineItems [Line Items] |  |  |
Average foreign exchange rate | 0.88 | 0.82 | 0.73
Closing foreign exchange rate | 0.89 | 0.86 | 0.73
Norwegian Krone [Member] |  |  |
DisclosureOfPrinciplesOfConsolidationLineItems [Line Items] |  |  |
Average foreign exchange rate | 9.33 | 9.29 | 8.95
Closing foreign exchange rate | 9.83 | 9.09 | 9.60
Australian dollar [Member] |  |  |
DisclosureOfPrinciplesOfConsolidationLineItems [Line Items] |  |  |
Average foreign exchange rate | 1.47 | 1.49 | 1.48
Closing foreign exchange rate | 1.53 | 1.46 | 1.49

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