Operating costs - Summary of general and administrative expenses (Details) - EUR (€) € in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expenses by nature [abstract]
Personnel costs and directors fees	€ (17,756)	€ (15,160)	€ (12,739)
Other operating expenses	(6,659)	(6,584)	(6,388)
General and administrative expenses	€ (24,415)	€ (21,744)	€ (19,127)

The amount of expense relating to general and administrative activities of the entity.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-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

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