Income taxes - Reconciliation (Details) - EUR (€) € in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of profit before income taxes to income tax
Loss before tax	€ (8,445)	€ (11,311)	€ (9,530)
Prevailing statutory tax rate (as a percent)	28.00%
Tax expense at prevailing statutory rate (28%)	€ 2,365	3,167	2,668
Non-deductible expenses	(326)	(170)	(47)
Non-taxable income	266	116
Tax-rate related differences	(139)	(128)	(259)
Unrecognized temporary differences and tax losses	(2,246)	(2,987)	(2,426)
Income tax benefit (expense)	€ (80)	€ (2)	€ (64)

The applicable income tax rate.

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The aggregate amount included in the determination of profit (loss) for the period in respect of current tax and deferred tax that relate to continuing operations. [Refer: Continuing operations [member]; Current tax expense (income); Deferred tax expense (income)]

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The profit (loss) from continuing operations before tax expense or income. [Refer: Profit (loss)]

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No definition available.

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to changes in the tax rate. [Refer: Accounting profit]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to expenses not deductible in determining taxable profit (tax loss). [Refer: Accounting profit]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to revenues that are exempt from taxation. [Refer: Accounting profit]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to tax losses. [Refer: Accounting profit]

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The product of the accounting profit multiplied by the applicable tax rate(s). [Refer: Accounting profit; Applicable tax rate]

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