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Separation Transaction and Discontinued Operations - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Nov. 01, 2016 | May 31, 2017 | Apr. 30, 2017 | Feb. 28, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Common stock sold |  |  |  | 23,353,000 | 12,958,767 |  |  |
Common stock sold per share |  |  |  | $ 38.03 | $ 35.91 |  |  |
Cash proceeds sale of common stock |  |  |  | $ 888 |  |  |  |
Pre-tax gain on sale of common stock |  |  |  | $ 351 |  |  | $ 167 |
Investment in common stock of Alcoa Corporation |  |  |  |  |  |  | 0 | $ 1,020
Investment Banks [Member] |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Common stock sold |  | 12,958,767 | 12,958,767 |  |  |  |  |
Common stock sold per share |  | $ 35.91 | $ 35.91 |  |  |  |  |
Pre-tax gain on sale of common stock |  | $ 167 | $ 167 |  |  |  |  |
Yadkin [Member] |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Proceeds from sale of assets |  |  |  |  | $ 5 | $ 238 | $ 243 |
Alcoa Corporation [Member] |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Common stock outstanding percentage | 80.10% |  |  |  |  |  |  |
Number of common stock retained |  |  |  |  |  |  | 36,311,767 |
Percentage of common stock retained |  |  |  |  |  |  | 19.90% |

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