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Financial Risk Management - Summary of Nominal Value of Commodity Derivatives (Detail) - Commodity price risk [member] € in Millions	12 Months Ended
	Dec. 31, 2018 EUR (€)
Aluminium [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Maturity period	2019-2023
Premiums [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Maturity period	2019-2021
Copper [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Maturity period	2019-2021
Silver [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Maturity period	2019-2020
Zinc [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Maturity period	2019-2021
Natural gas [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Maturity period	2019-2020
Less than 1 year [member] | Aluminium [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	€ 392
Less than 1 year [member] | Premiums [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	10
Less than 1 year [member] | Copper [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	5
Less than 1 year [member] | Silver [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	9
Less than 1 year [member] | Zinc [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	8
Less than 1 year [member] | Natural gas [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	4
Over 1 year [member] | Aluminium [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	48
Over 1 year [member] | Premiums [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	4
Over 1 year [member] | Copper [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	5
Over 1 year [member] | Zinc [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	17
Over 1 year [member] | Natural gas [member]
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Nominal amount of derivatives	€ 2

X

- Definition

Derivative instrument maturity period.

+ References

No definition available.

+ Details

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X

- Definition

The nominal amount of a credit derivative. [Refer: Derivatives [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 24G
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_24G_a&doctype=Standard
-URIDate 2018-03-16

+ Details

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X

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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X

- Details

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- Details

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- Details

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