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Contingencies and Commitments - Additional Information (Detail) |  | 1 Months Ended | 9 Months Ended | 15 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 19, 2009 EUR (€) Plant | Jul. 31, 2014 Plant | Jun. 30, 2012 EUR (€) Installment | Jun. 30, 2012 USD ($) | Sep. 30, 2015 EUR (€) | Dec. 31, 2013 EUR (€) | Dec. 31, 2013 USD ($) | Sep. 30, 2015 USD ($) | Jun. 05, 2015 USD ($) | Feb. 21, 2013 EUR (€) | Feb. 21, 2013 USD ($) | Dec. 31, 2012 t | Jun. 30, 2012 USD ($) Installment | Mar. 26, 2012 EUR (€) | Mar. 26, 2012 USD ($) | Sep. 30, 2010 t | Nov. 19, 2009 USD ($)
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claimed obligation, minimum |  |  | € 159,000,000 |  |  |  |  |  |  |  |  |  | $ 178,000,000 |  |  |  |
Claimed obligation, maximum |  |  | 303,000,000 |  |  |  |  |  |  |  |  |  | 340,000,000 |  |  |  |
Management estimate for maximum exposure from class action |  |  |  |  |  |  |  |  |  |  |  |  |  | € 85,000,000 | $ 110,000,000 |  |
Number of smelters | Plant |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  | 303,000,000 | $ 375,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in recovery amount |  |  | 53,000,000 |  |  |  |  |  |  |  |  |  | $ 65,000,000 |  |  |  |
Payments to Italian Government |  |  | € 250,000,000 | $ 310,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Italian Government, installments amount |  |  |  |  |  | € 50,000,000 | $ 69,000,000 |  |  |  |  |  |  |  |  |  |
Number of installments, litigation payment | Installment |  |  | 5 |  |  |  |  |  |  |  |  |  | 5 |  |  |  |
Litigation reserve |  |  | € 159,000,000 |  |  |  |  |  |  |  |  |  | $ 178,000,000 |  |  |  |
Amount of curtailment, metric-tons-per-year | t |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 44,000 |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for contract losses |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for contract losses |  |  |  |  |  |  |  |  |  | € 76,000,000 | 97,000,000 |  |  |  |  |  |
Italy [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of years operating under a power supply structure |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |
Charge related to European Commission announcement | € 173,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 250,000,000
Number of smelters | Plant | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Write-off of receivable | € 14,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,000,000
Establishment reserve | € 159,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 230,000,000
Portovesme [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of curtailment, metric-tons-per-year | t |  |  |  |  |  |  |  |  |  |  |  | 150,000 |  |  |  |  |
Alcoa [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in recovery amount |  |  |  |  | € 53,000,000 |  |  | $ 59,000,000 |  |  |  |  |  |  |  |  |
Noncurrent asset |  |  |  |  | € 91,000,000 |  |  | $ 103,000,000 |  |  |  |  |  |  |  |  |
Alcoa Trasformazioni [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management estimate for maximum exposure from class action |  |  |  |  |  |  |  |  |  | € 76,000,000 | $ 97,000,000 |  |  |  |  |  |
Lockheed Martin [Member] | Pending Litigation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claimed obligation, minimum |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |
Claimed obligation, maximum |  |  |  |  |  |  |  |  | $ 76,000,000 |  |  |  |  |  |  |  |

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