Contingencies and Commitments - Additional Information - 2 (Detail)	9 Months Ended				9 Months Ended						3 Months Ended	9 Months Ended	63 Months Ended			9 Months Ended		12 Months Ended		93 Months Ended								9 Months Ended				3 Months Ended		9 Months Ended		33 Months Ended		0 Months Ended		9 Months Ended	1 Months Ended		0 Months Ended			1 Months Ended				0 Months Ended					36 Months Ended		3 Months Ended	12 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2014 EUR (€)	Jun. 30, 2012 USD ($)	Jun. 30, 2012 EUR (€)	Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($) Entity	Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] SAR	Sep. 30, 2014 Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($)	Sep. 30, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($)	Jan. 31, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($)	Sep. 30, 2014 Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($)	Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($)	Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($) Agency	Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($)	Apr. 11, 2014 Maaden Alcoa Joint Venture [Member]	Dec. 31, 2013 Maaden Alcoa Joint Venture [Member] USD ($)	Sep. 30, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($)	Jan. 31, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($)	Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Sep. 30, 2011 Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Sep. 30, 2011 Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Sep. 30, 2014 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($)	Dec. 31, 2013 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($)	Sep. 30, 2014 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($)	Dec. 31, 2013 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($)	Sep. 30, 2014 State and Local Jurisdiction [Member] USD ($)	Sep. 30, 2014 State and Local Jurisdiction [Member] BRL	Sep. 30, 2014 Financial Guarantee [Member] Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($)	Sep. 30, 2014 Financial Guarantee [Member] Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($)	Sep. 30, 2014 Estreito Project [Member] USD ($) MWh	Sep. 30, 2014 Estreito Project [Member] BRL	Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Feb. 27, 2014 Ligestra [Member] USD ($)	Feb. 27, 2014 Ligestra [Member] EUR (€)	Sep. 30, 2014 Ligestra [Member]	Jan. 31, 2014 Ligestra [Member] Minimum [Member]	Jan. 31, 2014 Ligestra [Member] Maximum [Member]	Feb. 27, 2014 Italian Ministry of Environment and Protection of Land and Sea[Member] USD ($)	Feb. 27, 2014 Italian Ministry of Environment and Protection of Land and Sea[Member] EUR (€)	Apr. 08, 2013 Alcoa World Alumina Brasil [Member] BRL	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] USD ($)	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] BRL	May 31, 2012 Alcoa World Alumina Brasil [Member] USD ($)	May 31, 2012 Alcoa World Alumina Brasil [Member] BRL	Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Minimum [Member] USD ($)	Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Maximum [Member] USD ($)	Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Fixed Assets [Member] BRL	Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Fixed Assets [Member] Minimum [Member] USD ($)	Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Fixed Assets [Member] Maximum [Member] USD ($)	Dec. 31, 2002 Alcoa Aluminio [Member] t	Sep. 30, 2014 Maaden Company [Member] Maaden Alcoa Joint Venture [Member]	Jun. 30, 2014 Portovesme [Member] USD ($)	Dec. 31, 2009 Portovesme [Member] USD ($)	Dec. 31, 2012 Baie Comeau [Member] USD ($)	Dec. 31, 2012 Mosjoen [Member] USD ($)	Dec. 31, 2009 Fusina [Member] USD ($)
Loss Contingencies [Line Items]
Increase in remediation reserve	$ 42,000,000																																																								$ 3,000,000	$ 3,000,000	$ 25,000,000	$ 20,000,000	$ 12,000,000
Percentage of payments and remediation costs																																									50.00%	80.00%
Soil remediation project, estimated cost																																											33,000,000	24,000,000
Payments for emergency action and natural resource damages																																											25,000,000	18,000,000
Costs and payments																																						17,000,000	12,000,000
Costs and payments related to damages																																						9,000,000	6,000,000
Payment period																																								10 years
Total combined assessments	307,000,000	242,000,000																								51,000,000	125,000,000
Disallowed tax credits																																														110,000,000	220,000,000					175,000,000	0	70,000,000
Percentage of penalty of the gross disallowed amount																																														50.00%	50.00%
Value added tax receivable																																																41,000,000	82,000,000
Estimated range of reasonably possible loss, minimum																																													155,000,000					0	60,000,000
Administrative appeal, assessment period																																													2 years
Metal sold per month																																																							2,000
Ownership interest issued as guarantee for Saudi Arabian Joint Venture																																																								74.90%
Investment percentage in Australian Project											25.10%	25.10%	25.10%					20.00%	20.00%
Number of companies in Saudi Arabian joint venture					3	3
Amount invested by Alcoa and Aluminum Financing Limited					10,800,000,000	40,500,000,000						1,100,000,000																								40,000,000	40,000,000
Additional contributions											4,000,000	87,000,000	919,000,000					17,000,000	24,000,000	141,000,000	176,000,000											5,000,000	5,000,000	8,000,000	9,000,000	37,000,000	38,000,000
Equity method investment, carrying value											956,000,000	956,000,000	956,000,000		951,000,000
Capital investment							4,515,000,000	1,992,000,000	240,000,000	1,133,000,000						500,000,000	60,000,000
Debt service requirements, principal																												193,000,000	120,000,000
Debt service requirements, interest maximum																												60,000,000	30,000,000
Guarantee issued on behalf of smelting and rolling mill companies																						8,000,000	10,000,000	3,000,000	4,000,000
Percentage of amount required to be contributed in the event Alcoa would be required to make payments under the guarantees																40.00%
Number of agencies needed to downgrade Alcoa's credit rating below investment grade in order to require Alcoa to provide a letter of credit or fund an escrow account												2
Minimum percentage of equity commitment required											67.00%	67.00%	67.00%	67.00%
Investment percentage in Brazilian project																														25.49%	25.49%
Assured power from installed capacity																														150	150
Estimated project costs																														2,110,000,000	5,170,000,000
Aluminio's share of project																														540,000,000	1,320,000,000
Commitment expended on the project																														520,000,000	1,270,000,000
Prepayments made under the agreement for future gas transmission services																																312,000,000	358,000,000	312,000,000	358,000,000	312,000,000	358,000,000
Alcoa's maximum exposure to loss in projects			$ 385,000,000	€ 303,000,000																												$ 430,000,000	500,000,000	$ 430,000,000	500,000,000	$ 430,000,000	500,000,000