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Contingencies and Commitments - Additional Information (Detail) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2015 USD ($) | Dec. 31, 2015 EUR (€) | Feb. 21, 2013 USD ($) | Feb. 21, 2013 EUR (€) | Mar. 26, 2012 USD ($) | Mar. 26, 2012 EUR (€)
Loss Contingencies [Line Items] |  |  |  |  |  |
Management estimate for maximum exposure from class action |  |  |  |  | $ 110,000,000 | € 85,000,000
Partial reserve | $ 37,000,000 | € 34,000,000 |  |  |  |
Alcoa Trasformazioni [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Management estimate for maximum exposure from class action |  |  | $ 97,000,000 | € 76,000,000 |  |
Minimum [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Provision for contract losses |  |  | 0 |  |  |
Maximum [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Provision for contract losses |  |  | $ 97,000,000 | € 76,000,000 |  |

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