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Borrowings - Summary of Composition of Carrying Amounts of Total Borrowings (Detail) - EUR (€) € in Millions | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Disclosure of detailed information about borrowings [line items] |  |  |
Total borrowings | € 2,184 | € 2,127 | € 2,468
U.S. Dollar [member] |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |
Total borrowings | 1,442 | 1,387 |
Euro [member] |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |
Total borrowings | 722 | 720 |
Other currencies [member] |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |
Total borrowings | € 20 | € 20 |

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- Details
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