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Parent Company - Additional Information (Detail) - IFRS9 [member] - EUR (€) € in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Disclosure of condensed financial information of parent company [line items] |  |
Loss on opening retained earnings, net of tax | € 151 |
Loss allowance |  | € 0
Gain on other comprehensive income | € 53 |

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- Definition Line item represents disclosure of condensed financial information of parent company. + References No definition available. + Details
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- Definition Expected credit loss allowance. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Gain (loss) on opening retained earnings net. + References No definition available. + Details
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- Definition Gain (loss) on other comprehensive income. + References No definition available. + Details
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Namespace Prefix: | cstm_
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Balance Type: | credit
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- Details
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