Board of Directors - Share-Based Payment for Executive Board Members (Details) - Executive Board € in Thousands	12 Months Ended
Dec. 31, 2017 EUR (€) item	Dec. 31, 2016 EUR (€) item	Dec. 31, 2015 EUR (€) item
Share-Based Payment for Executive Board Members
Share-based payment \| €	€ 19,068	€ 14,233	€ 22,310
RSU
Share-Based Payment for Executive Board Members
Number of shares granted	117,929	147,041	192,345
PSU
Share-Based Payment for Executive Board Members
Number of shares granted	176,886	220,561

The number of instruments granted in share-based payment arrangement.

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Reference 2: http://www.xbrl.org/2003/role/exampleRef
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Balance Type:	na
Period Type:	duration