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Restructuring and Other Charges, Net - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020 | Dec. 31, 2019
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring and other charges, net (D) | $ 33 | $ 37 | $ 40 | $ 39 |  |
Restructuring reserve balance | 49 |  | 49 |  | $ 63 | $ 137
Payments for restructuring |  |  | 24 |  | 120 |
Noncurrent portion of the reserve | 1 |  | 1 |  | 1 |
Aviles and La Coruna Smelters [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring reserve balance |  |  |  |  | $ 30 |
Payments for restructuring | 7 |  | 13 |  |  |
Intalco (Washington) [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Curtailment of certain pension benefits |  | 27 |  | 27 |  |
Wenatchee (Washington) [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Curtailment of certain pension benefits |  | 11 |  | 13 |  |
Permanent Close [Member] | Suriname [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring and other charges (reversal) | (5) |  | (5) |  |  |
Permanent Close [Member] | Brazil [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring and other charges (reversal) | (5) |  | (5) |  |  |
Permanent Close [Member] | Tennessee [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring and other charges (reversal) |  |  | (12) |  |  |
Contract Termination [Member] | Wenatchee (Washington) and Intalco (Washington) [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring and other charges, net (D) | 3 |  | 9 |  |  |
Severance and Exit Costs [Member] | Warrick Rolling Mill [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Settlement and curtailment of certain other postretirement benefits |  |  | 9 |  |  |
Other Item Charges [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring and other charges, net (D) | 1 |  | 5 |  |  |
Pension Benefits [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring and other charges, net (D) | $ 39 |  | $ 39 |  |  |
Curtailment of certain pension benefits |  | $ (1) |  | $ (4) |  |

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