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Borrowings - Summary of Reconciliation of Movement in Net Borrowings Explanatory (Parenthetical) (Detail) € in Millions			6 Months Ended	12 Months Ended
	Nov. 09, 2017 EUR (€) EUR_per_USD	Feb. 16, 2017 EUR (€) EUR_per_USD	Jun. 30, 2017 EUR (€)	Dec. 31, 2017 EUR (€)
Disclosure of detailed information about borrowings [line items]
Proceeds from the issuance of senior notes			€ 610	€ 1,440
Redemption of senior notes			€ 610	€ 1,559
Constellium N.V. USD Senior Unsecured Notes (Issued November 2017, due 2026) [member]
Disclosure of detailed information about borrowings [line items]
Proceeds from the issuance of senior notes	€ 830
Exchange rate | EUR_per_USD	1.1630
Constellium N.V. Senior Unsecured Notes (Issued February 2017, due 2025) [member]
Disclosure of detailed information about borrowings [line items]
Proceeds from the issuance of senior notes		€ 610
Exchange rate | EUR_per_USD		1.0652
Senior secured and unsecured notes [member]
Disclosure of detailed information about borrowings [line items]
Redemption of senior notes	€ 949
Exchange rate | EUR_per_USD	1.1630

X

- Definition

The spot exchange rate at the end of the reporting period. Exchange rate is the ratio of exchange for two currencies. Spot exchange rate is the exchange rate for immediate delivery.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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The cash inflow from the issuing of bonds, notes and debentures.

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