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Investments Accounted for Under Equity Method - Summary of Share of Joint Ventures (Detail) - EUR (€) € in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of joint ventures [line items] |  |  |
Group share of joint venture's net assets | € 1 | € 1 | € 16
Group share of joint venture's profit / (loss) | € (33) | € (29) | € (14)
Constellium-Bowling Green [member] |  |  |
Disclosure of joint ventures [line items] |  |  |
% interest | 51.00% | 51.00% |
Group share of joint venture's net assets | € (49) | € (14) |
Group share of joint venture's profit / (loss) | € (33) | € (29) |
Rhenaroll S.A. [member] |  |  |
Disclosure of joint ventures [line items] |  |  |
% interest | 49.85% | 49.85% |
Group share of joint venture's net assets | € 1 | € 1 |
Group share [member] |  |  |
Disclosure of joint ventures [line items] |  |  |
Group share of joint venture's net assets | (48) | (13) |
Group share of joint venture's profit / (loss) | (33) | (29) |
Reclassified to non-current other financial assets [member] |  |  |
Disclosure of joint ventures [line items] |  |  |
Group share of joint venture's net assets | € 49 | € 14 |

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- Definition 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 8 -IssueDate 2018-01-01 -Paragraph 24 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2018-03-01&anchor=para_24_a&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 12 -IssueDate 2018-01-01 -Paragraph B16 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2018-03-01&anchor=para_B16&doctype=Appendix&seq=2 -URIDate 2018-03-16 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 54 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_54_e&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The proportion of ownership interest in a joint venture attributable to the entity. [Refer: Joint ventures [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 27 -IssueDate 2018-01-01 -Paragraph 17 -Subparagraph b -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2018-03-01&anchor=para_17_b_iii&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 27 -IssueDate 2018-01-01 -Paragraph 16 -Subparagraph b -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2018-03-01&anchor=para_16_b_iii&doctype=Standard -URIDate 2018-03-16 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 12 -IssueDate 2018-01-01 -Paragraph 21 -Subparagraph a -Clause iv -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2018-03-01&anchor=para_21_a_iv&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition 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)] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 85 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_85&doctype=Standard -URIDate 2018-03-16 + Details
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