Contingencies and Commitments - Additional Information - 2 (Detail) € in Millions, BRL in Millions, AUD in Millions, SAR in Billions						1 Months Ended								3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended							36 Months Ended	93 Months Ended
	Apr. 08, 2015 USD ($) Installment Refinery	Feb. 27, 2014 USD ($)	Feb. 27, 2014 EUR (€)	Apr. 08, 2013 USD ($)	Apr. 08, 2013 BRL	Apr. 30, 2016 Refinery	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jan. 31, 2014	Oct. 31, 2013 USD ($)	Mar. 31, 2013 USD ($)	May 31, 2012 USD ($)	May 31, 2012 BRL	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2016 AUD	Jun. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 SAR	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	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, 2016 EUR (€)	Sep. 30, 2016 BRL	Sep. 30, 2016 AUD	Dec. 31, 2015 AUD	Apr. 08, 2013 BRL	Mar. 31, 2013 BRL
Loss Contingencies [Line Items]
Increase in remediation reserve								$ 1,000,000						$ 13,000,000				$ 26,000,000
Revised project cost submitted to reflect the removal of a larger volume of contaminated soil							$ 7,000,000
Total combined assessments														296,000,000				296,000,000												€ 244
Number of alumina refineries to be powered under supplied agreement | Refinery						3
State and Local Jurisdiction [Member]
Loss Contingencies [Line Items]
Total combined assessments														43,000,000				43,000,000													BRL 140
Maaden Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Required plan contributions																		10,800,000,000	SAR 40.5
Capital investment																		1,000,000
Equity investments														863,000,000				863,000,000		$ 928,000,000
Maaden Alcoa Joint Venture [Member] | Arconic [Member]
Loss Contingencies [Line Items]
Required plan contributions																		1,100,000,000
Capital investment																		$ 982,000,000
Ma'aden Joint Venture [Member]
Loss Contingencies [Line Items]
Ownership interest in joint venture																		74.90%	74.90%
Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Ownership interest in joint venture						60.00%												25.10%	25.10%
Project financing Investment														0				$ 0
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Loss Contingencies [Line Items]
Project financing Investment														4,207,000,000				4,207,000,000
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														3,000,000				3,000,000		7,000,000
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] | Financial Guarantee [Member]
Loss Contingencies [Line Items]
Debt service requirements, principal																		100,000,000
Debt service requirements, interest maximum																		35,000,000
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] | Arconic [Member]
Loss Contingencies [Line Items]
Project financing Investment														1,056,000,000				1,056,000,000
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														3,000,000				3,000,000		$ 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 Mine And Refinery Company [Member] | AWAC [Member]
Loss Contingencies [Line Items]
Project financing Investment														560,000,000				560,000,000
Maaden Alcoa Joint Venture Mine And Refinery Company [Member] | Arconic [Member]
Loss Contingencies [Line Items]
Project financing Investment														2,232,000,000				$ 2,232,000,000
Alumina Limited [Member]
Loss Contingencies [Line Items]
Percentage of amount required to be contributed in the event Alcoa would be required to make payments under the guarantees																		40.00%	40.00%
Ligestra [Member]
Loss Contingencies [Line Items]
Costs and payments		$ 17,000,000	€ 12
Costs and payments related to damages		9,000,000	6
Payment period																		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		33,000,000	24
Payments for emergency action and natural resource damages		$ 25,000,000	€ 18
Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Disallowed tax credits											$ 110,000,000																								BRL 220
Percentage of penalty of the gross disallowed amount				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				32,000,000
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				$ 36,000,000
Alcoa Aluminio [Member]
Loss Contingencies [Line Items]
Metal sold per month | t																											2,000
Majority-Owned Subsidiary | AofA [Member] | Service Agreements [Member]
Loss Contingencies [Line Items]
Gas supply agreement period																				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 | AofA [Member] | Service Agreements [Member] | First Installment [Member]
Loss Contingencies [Line Items]
Gas supply agreement prepayment amount	300,000,000
Asset included in other noncurrent assets														504,000,000				$ 504,000,000		$ 288,000,000												AUD 654	AUD 395
Majority-Owned Subsidiary | AofA [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 Smelter [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve																							$ 25,000,000
Mosjoen [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve																	$ 11,000,000						$ 20,000,000
Dampier to Bunbury Natural Gas Pipeline [Member]
Loss Contingencies [Line Items]
Capital investment																									$ 17,000,000	AUD 24
Dampier to Bunbury Natural Gas Pipeline [Member] | AofA [Member]
Loss Contingencies [Line Items]
Capital investment																												$ 141,000,000	AUD 176
Investment percentage						30.00%																			20.00%	20.00%
Prepayments made under the agreement for future gas transmission services														$ 288,000,000				$ 288,000,000														AUD 373
Dampier to Bunbury Natural Gas Pipeline [Member] | Three-Year Equity Call Plan [Member]
Loss Contingencies [Line Items]
Required plan contributions																					39,000,000	AUD 40
Dampier to Bunbury Natural Gas Pipeline [Member] | New Equity Call Plan [Member]
Loss Contingencies [Line Items]
Required plan contributions															$ 3,000,000	AUD 5					$ 30,000,000	AUD 36
Capital investment															$ 20,000,000	AUD 27