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Investments Accounted for Under Equity Method - Summary of Share of Joint Ventures (Parenthetical) (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]
Revenue	€ 5,237	€ 4,743	€ 5,153
Constellium-UACJ ABS LLC [member]
Disclosure of investments accounted for using equity method [line items]
Ownership percentage	51.00%	51.00%
Revenue	€ 123	€ 15
Rhenaroll S.A. [member]
Disclosure of investments accounted for using equity method [line items]
Ownership percentage	49.85%	49.85%
Revenue	€ 3	€ 3

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

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

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