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Principles of consolidation (Details)	12 Months Ended
	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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- References

No definition available.

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- Definition

The average exchange rate used by the entity. Exchange rate is the ratio of exchange for two currencies.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 112
-Subparagraph c
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- Definition

The spot exchange rate at the end of the reporting period. Exchange rate is the ratio of exchange for two currencies. Spot exchange rate is the exchange rate for immediate delivery.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

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