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Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020
Pension and other postretirement benefits (L) |  |  |  |  |
Total Other comprehensive income (loss) | $ (256) | $ 125 | $ (388) | $ 87 |
Foreign currency translation |  |  |  |  |
Other comprehensive income | 255 | 229 | 19 | (679) |
Cash flow hedges (M) |  |  |  |  |
Net change from periodic revaluations | (397) | (513) | (700) | 339 |
Net amount reclassified to earnings | 67 | 13 | 119 | 38 |
Total Accumulated other comprehensive loss | (5,687) |  | (5,687) |  | $ (5,629)
Alcoa Corporation [Member] |  |  |  |  |
Pension and other postretirement benefits (L) |  |  |  |  |
Balance at beginning of period | (2,405) | (2,244) | (2,536) | (2,282) |
Unrecognized net actuarial loss and prior service cost/benefit | 164 | (181) | 233 | (201) |
Tax benefit |  | 4 | 2 | 10 |
Total Other comprehensive income (loss) before reclassifications, net of tax | 164 | (177) | 235 | (191) |
Amortization of net actuarial loss and prior service cost/benefit | 89 | 52 | 150 | 106 |
Tax expense |  | (2) | (1) | (4) |
Total amount reclassified from Accumulated other comprehensive loss, net of tax | 89 | 50 | 149 | 102 |
Total Other comprehensive income (loss) | 253 | (127) | 384 | (89) |
Balance at end of period | (2,152) | (2,371) | (2,152) | (2,371) |
Foreign currency translation |  |  |  |  |
Balance at beginning of period | (2,561) | (2,823) | (2,385) | (2,160) |
Other comprehensive income | 204 | 135 | 28 | (528) |
Balance at end of period | (2,357) | (2,688) | (2,357) | (2,688) |
Cash flow hedges (M) |  |  |  |  |
Balance at beginning of period | (912) | 169 | (708) | (532) |
Net change from periodic revaluations | (397) | (513) | (700) | 339 |
Tax benefit (expense) | 75 | 112 | 131 | (63) |
Total Other comprehensive (loss) income before reclassifications, net of tax | (322) | (401) | (569) | 276 |
Net amount reclassified to earnings | 67 | 13 | 119 | 38 |
Tax (expense) benefit | (11) | (2) | (20) | (3) |
Total amount reclassified from Accumulated other comprehensive loss, net of tax | 56 | 11 | 99 | 35 |
Total Other comprehensive (loss) income | (266) | (390) | (470) | 311 |
Balance at end of period | (1,178) | (221) | (1,178) | (221) |
Total Accumulated other comprehensive loss | (5,687) | (5,280) | (5,687) | (5,280) |
Alcoa Corporation [Member] | Aluminum Contracts [Member] |  |  |  |  |
Cash flow hedges (M) |  |  |  |  |
Net amount reclassified to earnings | 72 | 1 | 113 | 14 |
Alcoa Corporation [Member] | Financial Contracts [Member] |  |  |  |  |
Cash flow hedges (M) |  |  |  |  |
Net amount reclassified to earnings | (4) | 4 | 5 | 7 |
Alcoa Corporation [Member] | Interest Rate Contracts [Member] |  |  |  |  |
Cash flow hedges (M) |  |  |  |  |
Net amount reclassified to earnings | 1 | 1 | 4 | 2 |
Alcoa Corporation [Member] | Foreign Exchange Contract [Member] |  |  |  |  |
Cash flow hedges (M) |  |  |  |  |
Net amount reclassified to earnings | (2) | 7 | (3) | 15 |
Non-controlling Interest [Member] |  |  |  |  |
Pension and other postretirement benefits (L) |  |  |  |  |
Balance at beginning of period | (66) | (56) | (67) | (56) |
Unrecognized net actuarial loss and prior service cost/benefit | 2 | 1 | 2 |  |
Total Other comprehensive income (loss) before reclassifications, net of tax | 2 | 1 | 2 |  |
Amortization of net actuarial loss and prior service cost/benefit | 1 | 1 | 2 | 2 |
Total amount reclassified from Accumulated other comprehensive loss, net of tax | 1 | 1 | 2 | 2 |
Total Other comprehensive income (loss) | 3 | 2 | 4 | 2 |
Balance at end of period | (63) | (54) | (63) | (54) |
Foreign currency translation |  |  |  |  |
Balance at beginning of period | (904) | (1,079) | (844) | (834) |
Other comprehensive income | 51 | 94 | (9) | (151) |
Balance at end of period | (853) | (985) | (853) | (985) |
Cash flow hedges (M) |  |  |  |  |
Balance at beginning of period | (4) |  | (1) | 20 |
Net change from periodic revaluations | 9 | (4) | (1) | (30) |
Tax benefit (expense) | (3) | 1 |  | 8 |
Total Other comprehensive (loss) income before reclassifications, net of tax | 6 | (3) | (1) | (22) |
Net amount reclassified to earnings | (2) | 3 | 4 | 1 |
Tax (expense) benefit | 1 | (1) | (1) |  |
Total amount reclassified from Accumulated other comprehensive loss, net of tax | (1) | 2 | 3 | 1 |
Total Other comprehensive (loss) income | 5 | (1) | 2 | (21) |
Balance at end of period | 1 | (1) | 1 | (1) |
Total Accumulated other comprehensive loss | (915) | (1,040) | (915) | (1,040) |
Non-controlling Interest [Member] | Financial Contracts [Member] |  |  |  |  |
Cash flow hedges (M) |  |  |  |  |
Net amount reclassified to earnings | $ (2) | $ 3 | $ 4 | $ 1 |

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