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Investments Accounted for Under Equity Method - Summary of Share of Joint Ventures (Detail) - EUR (€) € in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of investments accounted for using equity method [line items]
Investments accounted for under the equity method	€ 1	€ 16	€ 30
Group share of joint venture's profit / (loss)	€ (29)	€ (14)	€ (3)
Constellium-UACJ ABS LLC [member]
Disclosure of investments accounted for using equity method [line items]
Ownership percentage	51.00%	51.00%
Investments accounted for under the equity method	€ (14)	€ 15
Group share of joint venture's profit / (loss)	€ (29)	€ (14)
Rhenaroll S.A. [member]
Disclosure of investments accounted for using equity method [line items]
Ownership percentage	49.85%	49.85%
Investments accounted for under the equity method	€ 1	€ 1
Group share [member]
Disclosure of investments accounted for using equity method [line items]
Investments accounted for under the equity method	(13)	16
Group share of joint venture's profit / (loss)	(29)	€ (14)
Non-current other financial assets [member]
Disclosure of investments accounted for using equity method [line items]
Investments accounted for under the equity method	€ 14

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- Definition

Disclosure Of Investments Accounted For Using Equity Method [Line Items]

+ References

No definition available.

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The amount of investments accounted for using the equity method. The equity method is a method of accounting whereby the investment is initially recognised at cost and adjusted thereafter for the post-acquisition change in the investor's share of net assets of the investee. The investor's profit or loss includes its share of the profit or loss of the investee. The investor's other comprehensive income includes its share of the other comprehensive income of the investee. [Refer: At cost [member]]

+ References

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- Definition

The proportion of ownership interest in a joint venture attributable to the entity. [Refer: Joint ventures [member]]

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The entity's share of the profit (loss) of joint ventures accounted for using the equity method. [Refer: Investments accounted for using equity method; Joint ventures [member]; Profit (loss)]

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