Contingencies and Commitments - Additional Information - 2 (Detail) € in Millions, SAR in Billions					1 Months Ended								3 Months Ended	9 Months Ended			12 Months Ended				36 Months Ended
	Feb. 27, 2014 EUR (€)	Feb. 27, 2014 USD ($)	Apr. 08, 2013 USD ($)	Apr. 08, 2013 BRL	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jan. 31, 2014	Oct. 31, 2013 USD ($)	Mar. 31, 2013 USD ($)	May 31, 2012 USD ($)	May 31, 2012 BRL	Sep. 30, 2017 USD ($)	Sep. 30, 2017 EUR (€)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 SAR	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2002 t	Sep. 30, 2017 USD ($)	Sep. 30, 2017 BRL	Apr. 08, 2013 BRL	Mar. 31, 2013 BRL
Loss Contingencies [Line Items]
Increase (decrease) in remediation reserve					$ 1,000,000								$ (5,000,000)		$ (2,000,000)
Payments related to remediation expenses applied against the reserve															$ 12,000,000		$ 24,000,000
Remediation expenses													4,000,000
Revised project cost submitted to reflect the removal of a larger volume of contaminated soil							$ 7,000,000
Remediation reserve balance																	324,000,000					$ 306,000,000
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%	40.00%
Tax Year 2006 Through 2009 [Member]
Loss Contingencies [Line Items]
Total combined assessments														€ 129								152,000,000
Ma'aden Joint Venture [Member]
Loss Contingencies [Line Items]
Ownership interest in joint venture														74.90%	74.90%	74.90%
Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Ownership interest in joint venture														25.10%	25.10%	25.10%
Ma'aden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Loss Contingencies [Line Items]
Project financing Investment																						4,012,000,000
Maaden Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Equity investments																	853,000,000					871,000,000
Expected project investment															$ 10,800,000,000	SAR 40.5
Contribution to joint venture as short term borrowings													23,000,000		131,000,000
Other Noncurrent Liabilities and Deferred Credits [Member] | Ma'aden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Loss Contingencies [Line Items]
Guarantee issued on behalf of smelting and rolling mill companies																	3,000,000					2,000,000
Other Noncurrent Liabilities and Deferred Credits [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Loss Contingencies [Line Items]
Guarantee issued on behalf of smelting and rolling mill companies																	3,000,000					3,000,000
State and Local Jurisdiction [Member]
Loss Contingencies [Line Items]
Total combined assessments																						46,000,000	BRL 145,000,000
Income tax settled																						8	BRL 25
Baie Comeau Smelter [Member]
Loss Contingencies [Line Items]
Increase (decrease) in remediation reserve																			$ 25,000,000
Fusina [Member]
Loss Contingencies [Line Items]
Increase (decrease) in remediation reserve									$ 0											$ 12,000,000
Remediation expenses														€ 7	$ 7,000,000
Portovesme [Member]
Loss Contingencies [Line Items]
Increase (decrease) in remediation reserve																		$ 3,000,000		$ 3,000,000
Mosjoen [Member]
Loss Contingencies [Line Items]
Increase (decrease) in remediation reserve						$ 11,000,000							(2,000,000)						$ 20,000,000
Remediation reserve balance																	8,000,000					4,000,000
East St. Louis, IL Site [Member]
Loss Contingencies [Line Items]
Remediation reserve balance																	$ 4,000,000					4,000,000
Long-term inspection, maintenance, and monitoring program period in years														30 years	30 years	30 years
Financial Guarantee [Member] | Ma'aden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Loss Contingencies [Line Items]
Debt service requirements, principal															$ 89,000,000
Debt service requirements, interest maximum															15,000,000
Financial Guarantee [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Loss Contingencies [Line Items]
Debt service requirements, principal															109,000,000
Debt service requirements, interest maximum															20,000,000
Alcoa Corporation [Member] | Ma'aden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Loss Contingencies [Line Items]
Project financing Investment																						1,007,000,000
Alcoa Corporation [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Loss Contingencies [Line Items]
Project financing Investment																						2,181,000,000
Alcoa Corporation [Member] | Maaden Alcoa Joint Venture [Member]
Loss Contingencies [Line Items]
Expected project investment															1,100,000,000
Capital investment commitment paid-to-date															982,000,000
Contribution to joint venture as short term borrowings													$ 8,000,000		$ 44,000,000
AWAC [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Loss Contingencies [Line Items]
Project financing Investment																						$ 547,000,000
Ligestra [Member]
Loss Contingencies [Line Items]
Costs and payments	€ 12	$ 17,000,000
Costs and payments related to damages	6	9,000,000
Payment period														10 years	10 years	10 years
Italian Ministry of Environment and Protection of Land and Sea [Member]
Loss Contingencies [Line Items]
Soil remediation project, estimated cost	24	33,000,000
Payments for emergency action and natural resource damages	€ 18	$ 25,000,000
Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Disallowed tax credits										$ 110,000,000															BRL 220,000,000
Percentage of penalty of the gross disallowed amount			50.00%	50.00%						50.00%
Value added tax receivable											$ 41,000,000	BRL 82,000,000
Alcoa Aluminio [Member]
Loss Contingencies [Line Items]
Metal sold per month | t																					2,000
Minimum [Member] | Ligestra [Member]
Loss Contingencies [Line Items]
Percentage of payments and remediation costs								50.00%
Minimum [Member] | Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Estimated range of reasonably possible loss, minimum			$ 0
Minimum [Member] | Alcoa World Alumina Brasil [Member] | Fixed Assets [Member]
Loss Contingencies [Line Items]
Disallowed tax credits			0
Maximum [Member] | Ligestra [Member]
Loss Contingencies [Line Items]
Percentage of payments and remediation costs								80.00%
Maximum [Member] | Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Estimated range of reasonably possible loss, minimum			32,000,000	BRL 103,000,000
Maximum [Member] | Alcoa World Alumina Brasil [Member] | Fixed Assets [Member]
Loss Contingencies [Line Items]
Disallowed tax credits			$ 37,000,000																					BRL 117,000,000