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Subsequent Events - Additional Information (Detail) $ / shares in Units, $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Oct. 02, 2017 $ / shares shares | Feb. 28, 2017 shares | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 shares | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Oct. 13, 2017 USD ($)
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Depository shares |  | 23,353,000 |  |  | 12,958,767 |  |  |  |
Non-operating income | $ |  |  |  | $ 1 |  | $ 11 | $ 526 | $ 40 |
Scenario, Forecast [Member] | Alcoa Corporation [Member] |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Non-operating income | $ |  |  | $ 25 |  |  |  |  |  |
Non cash non-operating income, after-tax | $ |  |  | $ 16 |  |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Depository shares | 24,022 |  |  |  |  |  |  |  |
Preferred stock converted to common stock | 31,428 |  |  |  |  |  |  |  |
Subsequent Event [Member] | Mandatory Convertible Preferred Stock [Member] |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Depository shares | 24,975,978 |  |  |  |  |  |  |  |
Interest in preferred stock | 0.1 |  |  |  |  |  |  |  |
Preferred stock conversion price | $ / shares | $ 1.56996 |  |  |  |  |  |  |  |
Preferred stock converted to common stock | 39,211,286 |  |  |  |  |  |  |  |
Subsequent Event [Member] | Alcoa Corporation [Member] | Electricity Contract [Member] |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |
Guarantees of third party related to project financing | $ |  |  |  |  |  |  |  |  | $ 485

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- Definition Preferred Stock Conversion Price + References No definition available. + Details
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- Definition The number of shares converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586 + Details
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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248 + Details
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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
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