Shareholders' equity - Conditional capital (Details) - EUR (€)	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Disclosure of classes of share capital [line items]
Increase in capital stock from exercise of stock options	€ 486,000	€ 4,096,000

The increase (decrease) in equity resulting from the exercise of options.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 106
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_106_d&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_IncreaseDecreaseThroughExerciseOfOptions
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration