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Separation Transaction and Discontinued Operations - Additional Information (Detail) |  |  |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 12 Months Ended |
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Oct. 02, 2017 shares | Nov. 01, 2016 USD ($) agreement | Oct. 20, 2016 shares | Feb. 28, 2017 USD ($) shares | Sep. 30, 2014 shares | May 31, 2017 USD ($) investment_bank $ / shares shares | Jun. 30, 2017 USD ($) $ / shares shares | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Feb. 14, 2017 $ / shares
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Investment in common stock of Alcoa Corporation |  |  |  |  |  |  |  |  | $ 0 | $ 1,020,000,000 |  |
Common stock sold (in shares) | shares | 24,022 |  |  |  | 25,000,000 |  | 12,958,767 |  |  |  |  |
Common stock sold per share (usd per share) | $ / shares |  |  |  |  |  |  | $ 35.91 |  |  |  |  |
Pre-tax gain on sale of common stock |  |  |  |  |  |  | $ 167,000,000 |  | 167,000,000 |  |  |
Proceeds from Alcoa Corporation's sale of Yadkin Hydroelectric Project |  |  |  |  |  |  |  |  | 245,000,000 | 16,000,000 | $ 25,000,000 |
Debt issuance and capital expenditures incurred |  |  |  |  |  |  |  |  |  | 110,000,000 |  |
Impairment charges |  | $ 0 |  |  |  |  |  |  | 58,000,000 | 80,000,000 | 0 |
Selling, General Administrative, and Other Expenses |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Costs related to separation transaction, pre-tax |  |  |  |  |  |  |  |  | 18,000,000 | 193,000,000 | 24,000,000 |
Costs related to separation transaction, after-tax |  |  |  |  |  |  |  |  | 12,000,000 | $ 158,000,000 | $ 24,000,000 |
Alcoa Corporation |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock outstanding percentage |  | 80.10% |  |  |  |  |  |  |  |  |  |
Percentage of common stock retained |  |  | 19.90% |  |  |  |  |  |  |  |  |
Common stock conversion ratio |  |  | 0.333 |  |  |  |  |  |  |  |  |
Number of common stock retained (in shares) | shares |  |  | 36,311,767 |  |  |  |  |  |  |  |  |
Common stock sold (in shares) | shares |  |  |  | 23,353,000 |  |  |  |  |  |  |  |
Common stock sold per share (usd per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  | $ 38.03
Cash proceeds sale of common stock |  |  |  |  |  |  |  | $ 888,000,000 |  |  |  |
Pre-tax gain on sale of common stock |  |  |  | $ 351,000,000 |  |  |  |  |  |  |  |
Number of counterparties | investment_bank |  |  |  |  |  | 2 |  |  |  |  |  |
Payment guarantees related to disposal group | agreement |  | 2 |  |  |  |  |  |  |  |  |  |
Proceeds from Alcoa Corporation's sale of Yadkin Hydroelectric Project |  |  |  |  |  |  | $ 5,000,000 | $ 238,000,000 | $ 243,000,000 |  |  |
Investment Banks |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock sold (in shares) | shares |  |  |  |  |  | 12,958,767 |  |  |  |  |  |
Common stock sold per share (usd per share) | $ / shares |  |  |  |  |  | $ 35.91 |  |  |  |  |  |
Pre-tax gain on sale of common stock |  |  |  |  |  | $ 167,000,000 |  |  |  |  |  |

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