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Pensions and Other Post-employment Benefit Obligations - Summary of Benefits Payments Expected Paid Either by Pension Funds or Directly to Beneficiaries (Detail) € in Millions | 12 Months Ended
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Dec. 31, 2017 EUR (€)
Statement of financial position [abstract] |
2018 | € 52
2019 | 52
2020 | 52
2021 | 55
2022 | 56
2023 to 2027 | € 286

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- Definition Estimated future benefit payments Five Fiscal Years Thereafter + References No definition available. + Details
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Balance Type: | debit
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- Definition Estimated future benefit payments next twelve months + References No definition available. + Details
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- Definition Estimated future benefit payments Year Five + References No definition available. + Details
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Balance Type: | debit
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- Definition Estimated future benefit payments Year Four + References No definition available. + Details
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Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Estimated future benefit payments Year Three + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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- Definition Estimated future benefit payments Year Two + References No definition available. + Details
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Balance Type: | debit
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- References No definition available. + Details
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