Contingencies and Commitments - Additional Information - 2 (Detail) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended							1 Months Ended					1 Months Ended		1 Months Ended			9 Months Ended				9 Months Ended		3 Months Ended	12 Months Ended			3 Months Ended		12 Months Ended	3 Months Ended	9 Months Ended				9 Months Ended		9 Months Ended				12 Months Ended				9 Months Ended				12 Months Ended		1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013 USD ($) t MW	Dec. 31, 2011 Agency	Dec. 31, 2010 USD ($) Agency	Dec. 31, 2012 USD ($)	Jun. 30, 2012 USD ($)	Jun. 30, 2012 EUR (€)	Sep. 30, 2013 Other Noncurrent Liabilities and Deferred Credits [Member] USD ($)	Dec. 31, 2012 Other Noncurrent Liabilities and Deferred Credits [Member] USD ($)	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] BRL	May 31, 2012 Alcoa World Alumina Brasil [Member] USD ($)	May 31, 2012 Alcoa World Alumina Brasil [Member] BRL	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] USD ($)	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] Fixed Assets [Member] BRL	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] Minimum [Member] USD ($)	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] Minimum [Member] Fixed Assets [Member] USD ($)	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] Maximum [Member] USD ($)	Mar. 31, 2013 Alcoa World Alumina Brasil [Member] Maximum [Member] Fixed Assets [Member] USD ($)	Dec. 31, 2010 Alcoa [Member] USD ($)	Sep. 30, 2013 Barra Grande [Member] USD ($) MWh	Sep. 30, 2013 Barra Grande [Member] BRL MWh	Dec. 31, 2012 Barra Grande [Member] USD ($)	Dec. 31, 2012 Barra Grande [Member] BRL	Sep. 30, 2013 Estreito Project [Member] USD ($) MWh	Sep. 30, 2013 Estreito Project [Member] BRL	Jun. 30, 2012 East St. Louis, IL Site [Member] USD ($)	Dec. 31, 2005 East St. Louis, IL Site [Member] USD ($)	Dec. 31, 2009 Fusina [Member] USD ($)	Dec. 31, 2009 Portovesme [Member] USD ($)	Sep. 30, 2012 Baie Comeau [Member] USD ($)	Sep. 30, 2012 Mosjoen [Member] USD ($)	Dec. 31, 2011 Refining and Mining Company [Member] USD ($)	Sep. 30, 2013 Alcoa [Member] USD ($)	Sep. 30, 2013 Alcoa [Member] USD ($)	Sep. 30, 2013 Alcoa [Member] SAR	Dec. 31, 2012 Alcoa [Member] USD ($)	Dec. 31, 2011 Alcoa [Member]	Sep. 30, 2013 Ma'aden Joint Venture [Member] Entity	Dec. 31, 2011 Ma'aden Joint Venture [Member]	Sep. 30, 2013 Alumina Limited [Member]	Sep. 30, 2013 Machadinho and Barra Grande [Member]	Sep. 30, 2013 Serrado Facao and Estreito Projects [Member]	Sep. 30, 2013 Machadinho [Member] MWh	Dec. 31, 2010 Smelting and Rolling Mill Companies [Member] USD ($)	Sep. 30, 2013 Smelting and Rolling Mill Companies [Member]	Sep. 30, 2013 Smelting and Rolling Mill Companies [Member] Other Noncurrent Liabilities and Deferred Credits [Member] USD ($)	Dec. 31, 2012 Smelting and Rolling Mill Companies [Member] Other Noncurrent Liabilities and Deferred Credits [Member] USD ($)	Sep. 30, 2013 Serra Do Facao Project [Member] USD ($) MWh	Sep. 30, 2013 Serra Do Facao Project [Member] BRL	Dec. 31, 2012 Serra Do Facao Project [Member] USD ($)	Dec. 31, 2012 Serra Do Facao Project [Member] BRL	Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Sep. 30, 2013 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] USD ($)	Sep. 30, 2013 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] AUD	Sep. 30, 2013 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] USD ($)	Sep. 30, 2013 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] AUD	Sep. 30, 2013 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] USD ($)	Sep. 30, 2012 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] USD ($)
Loss Contingencies [Line Items]
Remediation reserve increase (decrease)																									$ 14	$ 15	$ 12	$ 3	$ 25	$ 20
Disallowed tax credits									220			110	175		0		80
Percentage of penalty of the gross disallowed amount									50.00%
Value added tax receivable										41	82
Estimated range of reasonably possible loss, minimum									155					0		70
Administrative appeal, assessment period									2 years
Ownership interest issued as guarantee for smelting and rolling mill companies																																				25.10%	74.90%	74.90%						25.10%
Number of joint venture entities																																					3
Capital investment	1,908			1,860																											1,992	10,800	10,800	40,500									4,035
Amount invested by Alcoa and Aluminum Financing Limited																																1,100	1,100
Term of investment by Alcoa and Aluminum Financing Limited, years																																	5 years
Capital investment commitment paid-to-date																																820	820
Capital investment commitment paid in period																																71	159
Equity method investment, carrying value																																956	956		816
Guarantee issued on behalf of smelting and rolling mill companies							4	4										1,013													500														9	10
Project financing			480
Portion of project financing representing Alcoa interest in venture			120
Debt service requirements, principal																															60												121
Debt service requirements, interest maximum																															25												60
Number of agencies		2	2
Ownership interest in joint venture at fair market value																																							40.00%
Commitment to taking share of output in associated projects at cost, in years																																								30 years	26 years
Investment percentage																			42.18%	42.18%			25.49%	25.49%																		30.99%					34.97%	34.97%			20.00%	20.00%
Assured power from installed capacity																			160	160			150	150																		120					65	65
Total investment in project																			147	332	159	326																									89	200	98	200
Estimated project costs																							2,290	5,170																							450	1,000
Aluminio's share of project																							580	1,320																							160	350
Additional contributions																																															160	350			141	176	26	26	2	3	9	9
Increase in estimated costs to complete project																							130	270
Commitment expended on the project																							560	1,260
Percentage of power self-sufficiency upon completion of project with full operating capacity	70.00%
Total energy demand	690
Temporarily declined in energy demand	260
Partial capacity curtailments of combined smelters	128,000
Initial cash investment																																																			17	24
Required plan contributions																																																							40	40
Prepayments made under the agreement for future gas transmission services																																																					328	353	328	353	328
Alcoa's maximum exposure to loss in projects					$ 403	€ 303																																															$ 460	500	$ 460	500	$ 460