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Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Segment Reporting Information [Line Items] |  |  |  |
Segment Adjusted EBITDA | $ 625 | $ 185 | $ 1,194 | $ 560
Depreciation, depletion, and amortization | 155 | 146 | 331 | 310
Equity (loss) income | 27 | (20) | 35 | (24)
Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 3,354 | 2,680 | 6,770 | 5,630
Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 525 | 536 | 1,076 | 1,110
Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 2,829 | 2,144 | 5,694 | 4,520
Bauxite [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Segment Adjusted EBITDA | 41 | 131 | 100 | 251
Depreciation, depletion, and amortization | 32 | 30 | 89 | 64
Bauxite [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 218 | 311 | 461 | 617
Bauxite [Member] | Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 179 | 245 | 364 | 480
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 39 | 66 | 97 | 137
Alumina [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Segment Adjusted EBITDA | 124 | 88 | 351 | 281
Depreciation, depletion, and amortization | 50 | 37 | 96 | 86
Equity (loss) income | (1) | (8) | (6) | (17)
Alumina [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 1,031 | 892 | 2,155 | 1,935
Alumina [Member] | Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 343 | 289 | 707 | 625
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 688 | 603 | 1,448 | 1,310
Aluminum [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Segment Adjusted EBITDA | 460 | (34) | 743 | 28
Depreciation, depletion, and amortization | 73 | 79 | 146 | 160
Equity (loss) income | 28 | (12) | 41 | (7)
Aluminum [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 2,105 | 1,477 | 4,154 | 3,078
Aluminum [Member] | Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 3 | 2 | 5 | 5
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | $ 2,102 | $ 1,475 | $ 4,149 | $ 3,073

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