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Investments - Additional Information (Detail) AUD in Millions, $ in Millions | 1 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Apr. 30, 2016 USD ($) Refinery | Apr. 30, 2016 AUD Refinery | Sep. 30, 2016 USD ($) | Sep. 30, 2016 AUD | Sep. 30, 2015 USD ($) | Dec. 31, 2004 | Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Number of alumina refineries to be powered under supplied agreement | Refinery | 3 | 3 |  |  |  |  |
Sales of investments |  |  | $ 280 |  | $ 40 |  |
Alcoa Joint Venture [Member] |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Ownership interest in joint venture | 60.00% | 60.00% | 25.10% | 25.10% |  |  |
Alumina Limited of Australia [Member] |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Ownership interest in joint venture | 40.00% | 40.00% |  |  |  |  |
AofA [Member] |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Gain after tax and noncontrolling interest |  |  | $ 11 | AUD 15 |  |  |
Pretax gain on sale of investments |  |  | 27 | AUD 35 |  |  |
AofA [Member] | Dampier to Bunbury Natural Gas Pipeline [Member] |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Investment percentage in DBP to be sold | 20.00% | 20.00% |  |  |  |  |
Amount to be received by sale of investment percentage in DBP | $ 145 | AUD 192 |  |  |  |  |
Percentage of access to be maintained in DBNGP transmission capacity for gas supply | 30.00% | 30.00% |  |  |  | 20.00% |
Exchange Traded Fixed Income and Equity Securities [Member] |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Sales of investments |  |  | 130 |  |  |  |
Loss on sale of investment |  |  | 3 |  |  |  |
Loss on sale of investment, after tax |  |  | (2) |  |  |  |
Carrying value of available-for-sale-securities |  |  | $ 83 |  |  |  | $ 193

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