Shareholders' equity - Share options (Details) - EUR (€)	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Shareholders' equity
Compensation expenses	€ 1,511,000	€ 1,273,000
Compensation expenses recorded in general and administrative costs	873,000	627,000
Compensation expenses recorded in research and development costs	€ 638,000	€ 646,000

The amount of expense arising from share-based payment transactions in which the goods or services received did not qualify for recognition as assets.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2017-01-01
-Paragraph 51
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_51_a&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_ExpenseFromSharebasedPaymentTransactionsInWhichGoodsOrServicesReceivedDidNotQualifyForRecognitionAsAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration