Share-Based Compensation - Summary of Movements in Shares (Detail)	12 Months Ended
Dec. 31, 2018 EUR (€) shares	Dec. 31, 2017 EUR (€) shares
Performance-Based RSU [member]
Disclosure of movement of potential shares [line items]
Potential Shares, Beginning balance \| shares	3,257,840	2,066,835
Potential Shares, Granted \| shares	701,109	892,781
Potential Shares, Over performance \| shares	633,670	552,728
Potential Shares, Vested \| shares	(1,265,635)
Potential Shares, Forfeited \| shares	(241,820)	(254,504)
Potential Shares, Ending balance \| shares	3,085,164	3,257,840
Weighted- Average Grant-Date Fair Value per Share, Beginning balance \| €	€ 8.56	€ 7.50
Weighted- Average Grant-Date Fair Value per Share, Granted \| €	15.31	11.52
Weighted- Average Grant-Date Fair Value per Share, Over performance \| €	7.28	7.63
Weighted- Average Grant-Date Fair Value per Share, Vested \| €	7.09
Weighted- Average Grant-Date Fair Value per Share, Forfeited \| €	8.40	8.29
Weighted- Average Grant-Date Fair Value per Share, Ending balance \| €	€ 10.45	€ 8.56
Restricted Stock Units [member]
Disclosure of movement of potential shares [line items]
Potential Shares, Beginning balance \| shares	944,500	479,500
Potential Shares, Granted \| shares	595,687	703,180
Potential Shares, Vested \| shares	(155,000)	(125,000)
Potential Shares, Forfeited \| shares	(72,663)	(113,180)
Potential Shares, Ending balance \| shares	1,312,524	944,500
Weighted- Average Grant-Date Fair Value per Share, Beginning balance \| €	€ 7.76	€ 7.82
Weighted- Average Grant-Date Fair Value per Share, Granted \| €	10.23	7.50
Weighted- Average Grant-Date Fair Value per Share, Vested \| €	10.83	7.02
Weighted- Average Grant-Date Fair Value per Share, Forfeited \| €	8.57	7.27
Weighted- Average Grant-Date Fair Value per Share, Ending balance \| €	€ 8.47	€ 7.76
Equity Awards Plans [member]
Disclosure of movement of potential shares [line items]
Potential Shares, Beginning balance \| shares	95,340	94,881
Potential Shares, Granted \| shares	30,709	54,409
Potential Shares, Vested \| shares	(68,136)	(53,950)
Potential Shares, Ending balance \| shares	57,913	95,340
Weighted- Average Grant-Date Fair Value per Share, Beginning balance \| €	€ 5.20	€ 5.01
Weighted- Average Grant-Date Fair Value per Share, Granted \| €	10.27	6.09
Weighted- Average Grant-Date Fair Value per Share, Vested \| €	4.85	5.76
Weighted- Average Grant-Date Fair Value per Share, Ending balance \| €	€ 8.31	€ 5.20

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

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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
-Name IFRS
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-IssueDate 2018-01-01
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The number of other equity instruments (ie other than share options) forfeited in a share-based payment arrangement.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-IssueDate 2018-01-01
-Paragraph 45
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The number of other equity instruments (ie other than share options) outstanding in a share-based payment arrangement.

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