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Segment Information - Schedule of Operating Results and Assets of Alcoa's Reportable Segments (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | $ 5,774 | $ 6,350 | $ 17,615 | $ 18,805
Intersegment sales | 1,308 | 1,597 | 4,132 | 4,845
Total sales | 7,082 | 7,947 | 21,747 | 23,650
Equity income (loss) | (4) | (2) | (16) | 24
Depreciation, depletion, and amortization | 346 | 355 | 1,035 | 1,050
Income taxes | 82 | 156 | 284 | 570
After-tax operating income (ATOI) | 235 | 462 | 806 | 1,652
Segment Information [Member] | Alumina [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 764 | 879 | 2,289 | 2,615
Intersegment sales | 575 | 751 | 1,768 | 2,107
Total sales | 1,339 | 1,630 | 4,057 | 4,722
Equity income (loss) | 2 | 2 | 4 | 27
Depreciation, depletion, and amortization | 120 | 117 | 348 | 332
Income taxes | (22) | 42 | (29) | 146
After-tax operating income (ATOI) | (9) | 154 | 49 | 482
Segment Information [Member] | Primary Metals [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 1,794 | 2,124 | 5,542 | 6,249
Intersegment sales | 691 | 798 | 2,234 | 2,559
Total sales | 2,485 | 2,922 | 7,776 | 8,808
Equity income (loss) | (5) | (4) | (16) | (4)
Depreciation, depletion, and amortization | 130 | 137 | 398 | 420
Income taxes | (19) | 21 | (51) | 129
After-tax operating income (ATOI) | (14) | 110 | (7) | 513
Segment Information [Member] | Global Rolled Products [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 1,849 | 1,974 | 5,607 | 5,951
Intersegment sales | 42 | 48 | 130 | 179
Total sales | 1,891 | 2,022 | 5,737 | 6,130
Equity income (loss) | (1) |  | (4) |
Depreciation, depletion, and amortization | 57 | 61 | 171 | 179
Income taxes | 44 | 26 | 136 | 94
After-tax operating income (ATOI) | 98 | 60 | 289 | 240
Segment Information [Member] | Engineered Products and Solutions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 1,367 | 1,373 | 4,177 | 3,990
Total sales | 1,367 | 1,373 | 4,177 | 3,990
Equity income (loss) |  |  |  | 1
Depreciation, depletion, and amortization | 39 | 40 | 118 | 119
Income taxes | 79 | 67 | 228 | 201
After-tax operating income (ATOI) | $ 160 | $ 138 | $ 475 | $ 417

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