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Investments - Summary of Investments (Detail) - EUR (€) € in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Investment [Line Items] |  |
Investments, current portion | € 35,560 | € 44,855
Non-current investments | 12,319 | 16,945
Total investments | 47,879 | 61,800
Commercial paper [member] |  |
Investment [Line Items] |  |
Investments, current portion | 13,929 | 22,208
U.S. Treasury securities [member] |  |
Investment [Line Items] |  |
Investments, current portion | 2,411 | 6,733
Corporate fixed income bonds [member] |  |
Investment [Line Items] |  |
Investments, current portion | 17,465 | 14,185
Non-current investments | 12,319 | 16,945
Agency bond [member] |  |
Investment [Line Items] |  |
Investments, current portion | € 1,755 | € 1,729

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