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Segment Information - Schedule of Operating Results of Alcoa's Reportable Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Segment Reporting Information [Line Items] |  |  |  |
Total sales | $ 5,573 | $ 6,239 | $ 17,289 | $ 17,529
Equity loss | (24) | (15) | (66) | (83)
Depreciation, depletion, and amortization | 305 | 328 | 918 | 978
Income taxes | 153 | 245 | 644 | 554
After-tax operating income (ATOI) | 410 | 581 | 1,633 | 1,309
Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 6,459 | 7,498 | 20,420 | 21,320
Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 899 | 1,264 | 3,155 | 3,793
Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 5,560 | 6,234 | 17,265 | 17,527
Alumina [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Equity loss | (9) | (7) | (27) | (19)
Depreciation, depletion, and amortization | 71 | 100 | 228 | 297
Income taxes | 85 | 26 | 264 | 78
After-tax operating income (ATOI) | 212 | 62 | 648 | 192
Alumina [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 1,303 | 1,368 | 4,046 | 3,964
Alumina [Member] | Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 391 | 482 | 1,323 | 1,472
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 912 | 886 | 2,723 | 2,492
Primary Metals [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Equity loss | (7) |  | (15) | (45)
Depreciation, depletion, and amortization | 106 | 124 | 324 | 377
Income taxes | (49) | 95 | 14 | 114
After-tax operating income (ATOI) | (59) | 245 | 195 | 327
Primary Metals [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 1,728 | 2,595 | 6,088 | 7,130
Primary Metals [Member] | Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 479 | 730 | 1,733 | 2,182
Primary Metals [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 1,249 | 1,865 | 4,355 | 4,948
Global Rolled Products [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Equity loss | (8) | (8) | (24) | (19)
Depreciation, depletion, and amortization | 56 | 62 | 168 | 178
Income taxes | 28 | 26 | 89 | 73
After-tax operating income (ATOI) | 62 | 69 | 192 | 193
Global Rolled Products [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 1,556 | 1,978 | 4,915 | 5,602
Global Rolled Products [Member] | Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment sales | 29 | 52 | 99 | 139
Global Rolled Products [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 1,527 | 1,926 | 4,816 | 5,463
Engineered Products and Solutions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Depreciation, depletion, and amortization | 61 | 32 | 166 | 95
Income taxes | 71 | 78 | 228 | 234
After-tax operating income (ATOI) | 151 | 155 | 472 | 455
Engineered Products and Solutions [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 1,397 | 1,034 | 3,933 | 3,103
Engineered Products and Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | 1,397 | 1,034 | 3,933 | 3,103
Transportation and Construction Solutions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Depreciation, depletion, and amortization | 11 | 10 | 32 | 31
Income taxes | 18 | 20 | 49 | 55
After-tax operating income (ATOI) | 44 | 50 | 126 | 142
Transportation and Construction Solutions [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total sales | 475 | 523 | 1,438 | 1,521
Transportation and Construction Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Third-party sales | $ 475 | $ 523 | $ 1,438 | $ 1,521

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