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Contingencies and Commitments - Commitments (Details) | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2018 USD ($) | Oct. 31, 2017 agreement | Dec. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) agreement | Sep. 30, 2018 agreement | Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Purchase obligations due in 2019 | $ 770,000,000 |  |  | $ 770,000,000 |  |  |  |
Purchase obligations due in 2020 | 169,000,000 |  |  | 169,000,000 |  |  |  |
Purchase obligations due in 2021 | 27,000,000 |  |  | 27,000,000 |  |  |  |
Purchase obligations due in 2022 | 24,000,000 |  |  | 24,000,000 |  |  |  |
Purchase obligations due in 2023 | 8,000,000 |  |  | 8,000,000 |  |  |  |
Purchase obligations due thereafter | 5,000,000 |  |  | 5,000,000 |  |  |  |
Operating leases, expense |  |  |  | 144,000,000 | $ 113,000,000 | $ 110,000,000 |  |
Long-term operating leases, minimum annual rentals due in 2019 | 94,000,000 |  |  | 94,000,000 |  |  |  |
Long-term operating leases, minimum annual rentals due in 2020 | 74,000,000 |  |  | 74,000,000 |  |  |  |
Long-term operating leases, minimum annual rentals due in 2021 | 54,000,000 |  |  | 54,000,000 |  |  |  |
Long-term operating leases, minimum annual rentals due in 2022 | 40,000,000 |  |  | 40,000,000 |  |  |  |
Long-term operating leases, minimum annual rentals due in 2023 | 30,000,000 |  |  | 30,000,000 |  |  |  |
Long-term operating leases, minimum annual rentals due thereafter | 87,000,000 |  |  | 87,000,000 |  |  |  |
Guarantees of third party related to project financing | $ 37,000,000 |  |  | 37,000,000 |  |  |  |
Number of long term energy supply agreements | agreement |  |  |  |  |  |  | 2 |
Number of agreements terminated | agreement |  | 1 |  |  |  |  |  |
Other (expense) income, net |  |  |  | (79,000,000) | 486,000,000 | $ (41,000,000) |  |
Insurance policy duration | 1 year |  |  |  |  |  |  |
Insurance policy limit | $ 80,000,000 |  |  |  |  |  |  |
Letters of credit | 6,301,000,000 |  | $ 6,807,000,000 | 6,301,000,000 | 6,807,000,000 |  |  |
Total amount committed under outstanding surety bonds | 50,000,000 |  |  | 50,000,000 |  |  |  |
Amount of outstanding surety bonds relating to these liabilities | 25,000,000 |  |  | 25,000,000 |  |  |  |
Purchase Agreement |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Number of long term energy supply agreements | agreement |  |  |  |  |  | 2 |  |
Long-term purchase commitment, period |  |  |  |  |  | 1 year |  |
Guarantees of third party related to project financing |  |  |  |  |  | $ 245,000,000 |  |
Alcoa Corporation Workers Compensation Claims |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Letters of credit, total amount committed |  |  |  |  |  |  |  | $ 54,000,000
Bank Loan Obligations | Letter of Credit |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Letters of credit | 136,000,000 |  |  | 136,000,000 |  |  |  |
Separation Agreement |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Combined fair value of guarantees | 6,000,000 |  | 8,000,000 | 6,000,000 | 8,000,000 |  |  |
Other (expense) income, net |  |  | 25,000,000 |  |  |  |  |
Total amount committed under outstanding surety bonds |  |  | 25,000,000 |  | 25,000,000 |  |  |
Separation Agreement | Other Noncurrent Liabilities and Deferred Credits |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Guarantees of third party related to project financing | $ 1,087,000,000 |  | $ 1,297,000,000 | $ 1,087,000,000 | $ 1,297,000,000 |  |  |

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