Contingencies and Commitments - Additional Information - 2 (Detail) BRL in Millions, SAR in Billions						1 Months Ended					3 Months Ended				9 Months Ended				12 Months Ended						36 Months Ended	93 Months Ended
	Apr. 08, 2015 USD ($) Installment Refinery	Feb. 27, 2014 EUR (€)	Feb. 27, 2014 USD ($)	Apr. 08, 2013 USD ($)	Apr. 08, 2013 BRL	Jan. 31, 2014	Oct. 31, 2013 USD ($)	Mar. 31, 2013 USD ($)	May. 31, 2012 USD ($)	May. 31, 2012 BRL	Sep. 30, 2015 USD ($)	Sep. 30, 2015 AUD	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) MWh Entity	Sep. 30, 2015 BRL MWh Entity	Sep. 30, 2015 AUD MWh Entity	Sep. 30, 2015 SAR MWh Entity	Dec. 31, 2014 USD ($)	Dec. 31, 2014 AUD	Dec. 31, 2012 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2004 USD ($)	Dec. 31, 2004 AUD	Dec. 31, 2002 t	Sep. 30, 2011 USD ($)	Sep. 30, 2011 AUD	Sep. 30, 2015 EUR (€)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 BRL	Sep. 30, 2015 AUD	Apr. 08, 2013 BRL	Mar. 31, 2013 BRL	Jun. 30, 2012 EUR (€) Installment	Jun. 30, 2012 USD ($) Installment
Loss Contingencies [Line Items]
Increase in remediation reserve											$ 1,000,000
Revised project cost submitted to reflect the removal of a larger volume of contaminated soil														$ 7,000,000
Total combined assessments																												€ 239,000,000	$ 268,000,000
Range of reasonable possible loss, maximum																																		€ 303,000,000	$ 340,000,000
Number of installments | Installment																																		5	5
State and Local Jurisdiction [Member]
Loss Contingencies [Line Items]
Total combined assessments																													32,000,000	BRL 133
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Loss Contingencies [Line Items]
Capital investment																													560,000,000
Percentage of amount required to be contributed in the event Alcoa would be required to make payments under the guarantees															40.00%	40.00%	40.00%	40.00%
Maaden Alcoa Joint Venture Mine And Refinery Company [Member] | Other Noncurrent Liabilities and Deferred Credits [Member]
Loss Contingencies [Line Items]
Guarantee issued on behalf of smelting and rolling mill companies																			$ 4,000,000										$ 3,000,000
Maaden Alcoa Joint Venture Mine And Refinery Company [Member] | Financial Guarantee [Member]
Loss Contingencies [Line Items]
Debt service requirements, principal															$ 120,000,000
Debt service requirements, interest maximum															$ 30,000,000
Maaden Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Ownership interest issued as guarantee for Saudi Arabian Joint Venture																												25.10%	25.10%	25.10%	25.10%
Number of companies in Saudi Arabian joint venture | Entity															1	1	1	1
Amount invested by Alcoa and Aluminum Financing Limited															$ 10,800,000,000			SAR 40.5
Capital investment											6,000,000				15,000,000
Equity method investment, carrying value																			983,000,000										$ 935,000,000
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Loss Contingencies [Line Items]
Capital investment																													1,098,000,000
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] | Other Noncurrent Liabilities and Deferred Credits [Member]
Loss Contingencies [Line Items]
Guarantee issued on behalf of smelting and rolling mill companies																			8,000,000										7,000,000
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] | Financial Guarantee [Member]
Loss Contingencies [Line Items]
Debt service requirements, principal															176,000,000
Debt service requirements, interest maximum															50,000,000
Alcoa [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Loss Contingencies [Line Items]
Capital investment																													2,232,000,000
Alcoa [Member] | Maaden Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Amount invested by Alcoa and Aluminum Financing Limited															1,100,000,000
Capital investment															$ 967,000,000
Alcoa [Member] | Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Loss Contingencies [Line Items]
Capital investment																													$ 4,374,000,000
Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Disallowed tax credits								$ 110,000,000																									BRL 220
Percentage of penalty				50.00%	50.00%			50.00%
Value added tax receivable									$ 41,000,000	BRL 82
Estimated range of reasonably possible loss, minimum | BRL					BRL 103
Alcoa World Alumina Brasil [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated range of reasonably possible loss, minimum				$ 0
Alcoa World Alumina Brasil [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated range of reasonably possible loss, minimum				25,000,000
Alcoa World Alumina Brasil [Member] | Fixed Assets [Member]
Loss Contingencies [Line Items]
Disallowed tax credits | BRL																																BRL 117
Alcoa World Alumina Brasil [Member] | Fixed Assets [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Disallowed tax credits				0
Alcoa World Alumina Brasil [Member] | Fixed Assets [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Disallowed tax credits				$ 28,000,000
Alcoa Aluminio [Member]
Loss Contingencies [Line Items]
Metal sold per month | t																									2,000
Maaden Company [Member] | Maaden Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Percent of guarantee issued by co-venturing entity																												74.90%	74.90%	74.90%	74.90%
Estreito Project [Member]
Loss Contingencies [Line Items]
Investment percentage															25.49%	25.49%	25.49%	25.49%
Assured power from installed capacity | MWh															150	150	150	150
Estimated project costs															$ 1,260,000,000	BRL 5,170
Aluminio's share of project															320,000,000	1,320
Commitment expended on the project															$ 310,000,000	BRL 1,270
Ligestra [Member]
Loss Contingencies [Line Items]
Costs and payments		€ 12,000,000	$ 17,000,000
Costs and payments related to damages		6,000,000	9,000,000
Payment period															10 years	10 years	10 years	10 years
Ligestra [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Percentage of payments and remediation costs						50.00%
Ligestra [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Percentage of payments and remediation costs						80.00%
Italian Ministry of Environment and Protection of Land and Sea[Member]
Loss Contingencies [Line Items]
Soil remediation project, estimated cost		24,000,000	33,000,000
Payments for emergency action and natural resource damages		€ 18,000,000	$ 25,000,000
Majority-Owned Subsidiary | Alcoa Of Australia Limited [Member] | Service Agreements [Member]
Loss Contingencies [Line Items]
Gas supply agreement period															12 years	12 years	12 years	12 years
Number of alumina refineries to be powered under supplied agreement | Refinery	3
Number of installments | Installment	2
Gas supply agreement prepayment amount	$ 500,000,000
Majority-Owned Subsidiary | Alcoa Of Australia Limited [Member] | Service Agreements [Member] | First Installment [Member]
Loss Contingencies [Line Items]
Gas supply agreement prepayment amount	300,000,000
Majority-Owned Subsidiary | Alcoa Of Australia Limited [Member] | Service Agreements [Member] | Second Installment [Member]
Loss Contingencies [Line Items]
Gas supply agreement prepayment amount	$ 200,000,000
Fusina [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve							$ 0															$ 12,000,000
Portovesme [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve																			3,000,000			$ 3,000,000
Baie Comeau [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve																					$ 25,000,000
Mosjoen [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve											1,000,000		$ 11,000,000		$ 12,000,000						$ 20,000,000
Dampier to Bunbury Natural Gas Pipeline [Member]
Loss Contingencies [Line Items]
Capital investment																							$ 17,000,000	AUD 24,000,000		$ 141,000,000	AUD 176,000,000
Investment percentage																							20.00%	20.00%
Dampier to Bunbury Natural Gas Pipeline [Member] | Three-Year Equity Call Plan [Member]
Loss Contingencies [Line Items]
Amount invested by Alcoa and Aluminum Financing Limited																			$ 39,000,000	AUD 40,000,000
Capital investment											$ 3,000,000	AUD 4,000,000			11,000,000		AUD 15,000,000
Prepayments made under the agreement for future gas transmission services																													$ 256,000,000		AUD 366,000,000
Range of reasonable possible loss, maximum																													$ 370,000,000		AUD 520,000,000
Dampier to Bunbury Natural Gas Pipeline [Member] | New Equity Call Plan [Member]
Loss Contingencies [Line Items]
Amount invested by Alcoa and Aluminum Financing Limited															30,000,000		36,000,000
Capital investment															$ 12,000,000		AUD 16,000,000