Debt (Bilateral Credit Facilities) (Details) - Line of Credit - Bilateral credit facilities	Dec. 31, 2018 EUR (€)
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	€ 320,000,000
Bilateral credit facility expiring in May 2019, tranche one
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	80,000,000
Bilateral credit facility expiring in June 2019
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	60,000,000
Bilateral credit facility expiring in May 2021
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	100,000,000
Bilateral Credit Facility Expiring in May 2019 (2) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	€ 80,000,000