Debt (Bilateral Credit Facilities) (Details) - Line of Credit - Bilateral Credit Facility	Sep. 30, 2018 EUR (€)
Line of Credit Facility
Maximum borrowing capacity	€ 320,000,000
Bilateral Credit Facility Expiring in May 2019
Line of Credit Facility
Maximum borrowing capacity	80,000,000
Second Bilateral Credit Facility Expiring in May 2019
Line of Credit Facility
Maximum borrowing capacity	80,000,000
Bilateral Credit Facility Expiring in June 2019
Line of Credit Facility
Maximum borrowing capacity	60,000,000
Bilateral Credit Facility Expiring in May 2021
Line of Credit Facility
Maximum borrowing capacity	€ 100,000,000