Share-Based Compensation - Summary of Weighted-Average Fair Value (Detail)	12 Months Ended
Dec. 31, 2017 EUR (€) shares	Dec. 31, 2016 EUR (€) shares
Performance-Based RSU [member]
Disclosure of movement of potential shares [line items]
Potential Shares, Beginning balance \| shares	2,066,835	1,007,000
Potential Shares, Granted \| shares	892,781	1,292,000
Potential Shares, Over performance \| shares	552,728	47,229
Potential Shares, Forfeited \| shares	(254,504)	(279,394)
Potential Shares, Ending balance \| shares	3,257,840	2,066,835
Weighted- Average Grant-Date Fair Value per Share, Beginning balance \| €	€ 7.50	€ 7.77
Weighted- Average Grant-Date Fair Value per Share, Granted \| €	11.52	7.56
Weighted- Average Grant-Date Fair Value per Share, Over performance \| €	7.63	7.10
Weighted- Average Grant-Date Fair Value per Share, Forfeited \| €	8.29	8.71
Weighted- Average Grant-Date Fair Value per Share, Ending balance \| €	€ 8.56	€ 7.50
Restricted Stock Units [member]
Disclosure of movement of potential shares [line items]
Potential Shares, Beginning balance \| shares	479,500	269,500
Potential Shares, Granted \| shares	703,180	340,300
Potential Shares, Vested \| shares	(125,000)	(87,300)
Potential Shares, Forfeited \| shares	(113,180)	(43,000)
Potential Shares, Ending balance \| shares	944,500	479,500
Weighted- Average Grant-Date Fair Value per Share, Beginning balance \| €	€ 7.82	€ 16.10
Weighted- Average Grant-Date Fair Value per Share, Granted \| €	7.50	5.40
Weighted- Average Grant-Date Fair Value per Share, Vested \| €	7.02	19.75
Weighted- Average Grant-Date Fair Value per Share, Forfeited \| €	7.27	16.40
Weighted- Average Grant-Date Fair Value per Share, Ending balance \| €	€ 7.76	€ 7.82
Equity Awards Plans [member]
Disclosure of movement of potential shares [line items]
Potential Shares, Beginning balance \| shares	94,881	34,865
Potential Shares, Granted \| shares	54,409	81,858
Potential Shares, Vested \| shares	(53,950)	(21,842)
Potential Shares, Ending balance \| shares	95,340	94,881
Weighted- Average Grant-Date Fair Value per Share, Beginning balance \| €	€ 5.01	€ 14.11
Weighted- Average Grant-Date Fair Value per Share, Granted \| €	6.09	4.02
Weighted- Average Grant-Date Fair Value per Share, Vested \| €	5.76	15.84
Weighted- Average Grant-Date Fair Value per Share, Ending balance \| €	€ 5.20	€ 5.01

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Weighted average grant date fair value per share other equity instruments, granted.

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The number of other equity instruments (ie other than share options) granted in a share-based payment arrangement.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-IssueDate 2017-01-01
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