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Contingencies and Commitments - Commitments (Detail) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) agreement guarantee | Dec. 31, 2015 USD ($) | Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items] |  |  |  |  |
Purchase obligations due in 2018 | $ 861,000,000 | $ 861,000,000 |  |  |
Purchase obligations due in 2019 | 176,000,000 | 176,000,000 |  |  |
Purchase obligations due in 2020 | 74,000,000 | 74,000,000 |  |  |
Purchase obligations due in 2021 | 13,000,000 | 13,000,000 |  |  |
Purchase obligations due in 2022 | 6,000,000 | 6,000,000 |  |  |
Purchase obligations due thereafter | 5,000,000 | 5,000,000 |  |  |
Operating leases, expense |  | 113,000,000 | $ 110,000,000 | $ 112,000,000 |
Long-term operating leases, minimum annual rentals due in 2018 | 108,000,000 | 108,000,000 |  |  |
Long-term operating leases, minimum annual rentals due in 2019 | 88,000,000 | 88,000,000 |  |  |
Long-term operating leases, minimum annual rentals due in 2020 | 62,000,000 | 62,000,000 |  |  |
Long-term operating leases, minimum annual rentals due in 2021 | 46,000,000 | 46,000,000 |  |  |
Long-term operating leases, minimum annual rentals due in 2022 | 38,000,000 | 38,000,000 |  |  |
Long-term operating leases, minimum annual rentals due thereafter | 100,000,000 | 100,000,000 |  |  |
Guarantees of third party related to project financing | 29,000,000 | 29,000,000 |  |  |
Other income, net |  | 640,000,000 | 94,000,000 | $ 28,000,000 |
Letters of credit, total amount committed | 120,000,000 | 120,000,000 |  |  |
Total amount committed under outstanding surety bonds | 54,000,000 | 54,000,000 |  |  |
Other Noncurrent Liabilities and Deferred Credits |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Combined fair value of guarantees | 8,000,000 | 8,000,000 | 35,000,000 |  |
Separation Agreement |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Guarantees of third party related to project financing | 197,000,000 | 197,000,000 | 354,000,000 |  |
Other income, net | 25,000,000 |  |  |  |
Combined fair value of guarantees | 1,297,000,000 | 1,297,000,000 |  |  |
Total amount committed under outstanding surety bonds | $ 25,000,000 | $ 25,000,000 |  |  | $ 25,000,000
Alcoa Corporation Energy Supply Contracts Guarantee |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Guarantees of third party related to project financing |  |  | $ 50,000,000 |  |
Guarantor obligations, number of contracts | guarantee |  |  | 2 |  |
Alcoa Corporation Environmental Liabilities Guarantee |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Guarantees of third party related to project financing |  |  | $ 53,000,000 |  |
Purchase Agreement |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Long-term purchase commitment, period |  |  | 1 year |  |
Guarantees of third party related to project financing |  |  | $ 245,000,000 |  |
Number of long term energy supply agreements | agreement |  |  | 2 |  |
Alcoa Corporation Workers Compensation Claims |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Letters of credit, total amount committed |  |  |  |  | 62,000,000
Alcoa Corporation Equipment Leases And Energy Contracts |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Letters of credit, total amount committed |  |  |  |  | $ 103,000,000

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