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Operating Segment Information - Summary of Segment adjusted EBITDA and reconciliation of Adjusted EBITDA to Net Income (Parenthetical) (Detail) - EUR (€) € in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of segment adjusted EBITDA and reconciliation of adjusted EBITDA to net income [line items] |  |  |
Start-up costs and development cost | € (17) | € (25) | € (21)
Wise one-time cost |  | (20) | (38)
Cash received from Wise purchase price adjustment |  | 21 |
Wise purchase price adjustment |  | 20 |
Gains / (Losses) on pension plan amendments | 20 |  | € (5)
Automotive Body Sheet Growth Projects [member] |  |  |
Disclosure of segment adjusted EBITDA and reconciliation of adjusted EBITDA to net income [line items] |  |  |
Start-up costs and development cost |  | € (20) |
AS&I [member] |  |  |
Disclosure of segment adjusted EBITDA and reconciliation of adjusted EBITDA to net income [line items] |  |  |
Start-up costs and development cost | (16) |  |
Collective Bargaining agreement renegotiation [member] |  |  |
Disclosure of segment adjusted EBITDA and reconciliation of adjusted EBITDA to net income [line items] |  |  |
Legal fees and lump-sum payments | € 3 |  |
Collective bargaining agreement term | 5 years |  |

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- Definition Acquisition purchase price adjustment. + References No definition available. + Details
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Period Type: | duration

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- Definition Cash payment received upon acquisition. + References No definition available. + Details
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- Definition Collective bargaining agreements term. + References No definition available. + Details
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- Definition Disclosure of segment adjusted EBITDA and reconciliation of adjusted EBITDA to net income. + References No definition available. + Details
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- Definition Gains (losses) on pension plan amendments. + References No definition available. + Details
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Namespace Prefix: | cstm_
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Balance Type: | credit
Period Type: | duration

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- Definition Legal fees. + 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 One time costs. + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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- Definition Startup and development costs. + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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- Details
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- Details
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- Details
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Period Type: |