Long-term debt and capital lease obligations - Bonds (Details) - EUR (€)	Jun. 30, 2018	Dec. 31, 2017
Long-term Debt and Capital Lease Obligations.
Outstanding borrowings	€ 6,387,717,000	€ 6,678,407,000
Bonds
Long-term Debt and Capital Lease Obligations.
Outstanding borrowings	€ 3,896,771,000	€ 3,810,483,000

The amount of outstanding funds that the entity is obligated to repay.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_55&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_Borrowings
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant