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Separation Transaction and Discontinued Operations - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions |  |  | 3 Months Ended |
--- | --- | --- | --- | ---
Feb. 14, 2017 | Nov. 01, 2016 | Jun. 30, 2017 | Mar. 31, 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 |  |  |  |
Common stock sold per share | $ 38.03 |  |  |  |
Cash proceeds sale of common stock | $ 888 |  |  |  |
Pre-tax gain on sale of common stock | $ 351 |  |  |  |
Investment in common stock of Alcoa Corporation |  |  |  | $ 446 | $ 1,020
Yadkin [Member] |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Proceeds from sale of assets |  |  |  | $ 238 |
Scenario, Forecast [Member] |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Pre-tax gain on sale of common stock |  |  | $ 167 |  |
Scenario, Forecast [Member] | Yadkin [Member] |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Proceeds from sale of assets |  |  | $ 5 |  |
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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