Warrant plans - Rollforward (Details)	12 Months Ended
Dec. 31, 2019 EquityInstruments € / shares	Dec. 31, 2018 EquityInstruments € / shares	Dec. 31, 2017 EquityInstruments € / shares	Dec. 31, 2016 EquityInstruments € / shares
Warrant plans
Outstanding at the beginning of the year \| EquityInstruments	4,626,782	3,970,807	3,466,407
Granted during the period \| EquityInstruments	1,699,690	1,235,245	873,000
Forfeited during the year \| EquityInstruments	(30,750)	(12,000)
Exercised during the period \| EquityInstruments	(754,605)	(567,270)	(368,200)
Expired during the year \| EquityInstruments			(400)
Outstanding at the end of the year \| EquityInstruments	5,541,117	4,626,782	3,970,807
Exercisable \| EquityInstruments	1,139,682	882,734	763,344	669,704
Weighted average exercise price at beginning of the year \| € / shares	€ 53.30	€ 39.32	€ 27.1
Granted during the period \| € / shares	95.1	79.9	77.5
Forfeited during the year \| € / shares	88.9	43.2
Exercised during the period \| € / shares	22.8	13.5	14.4
Expired during the year \| € / shares			19.4
Weighted average exercise price at end of the year \| € / shares	70.09	53.30	39.32
Exercisable \| € / shares	€ 30.2	€ 14.0	€ 13.7	€ 10.3

The number of other equity instruments (ie other than share options) exercised in a share-based payment arrangement.

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+ References

No definition available.

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- Definition

The number of other equity instruments (ie other than share options) granted in a share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2019-01-01
-Paragraph 47
-Subparagraph b
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-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 2
-IssueDate 2019-01-01
-Paragraph 45
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_45&doctype=Standard
-URIDate 2019-03-27

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The number of other equity instruments (ie other than share options) exercisable in a share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 2
-IssueDate 2019-01-01
-Paragraph 45
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_45&doctype=Standard
-URIDate 2019-03-27

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The number of other equity instruments (ie other than share options) expired in a share-based payment arrangement.

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The number of other equity instruments (ie other than share options) forfeited in a share-based payment arrangement.

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The number of other equity instruments (ie other than share options) outstanding in a share-based payment arrangement.

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The weighted average exercise price of other equity instruments (ie other than share options) exercisable in a share-based payment arrangement. [Refer: Weighted average [member]]

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The weighted average exercise price of other equity instruments (ie other than share options) exercised or vested in a share-based payment arrangement. [Refer: Weighted average [member]]

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The weighted average exercise price of other equity instruments (ie other than share options) expired in a share-based payment arrangement. [Refer: Weighted average [member]]

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The weighted average exercise price of other equity instruments (ie other than share options) forfeited in a share-based payment arrangement. [Refer: Weighted average [member]]

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The weighted average exercise price of other equity instruments (ie other than share options) granted in a share-based payment arrangement. [Refer: Weighted average [member]]

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The weighted average exercise price of other equity instruments (ie other than share options) outstanding in a share-based payment arrangement. [Refer: Weighted average [member]]

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