Contingencies and Commitments - Additional Information - 2 (Detail)	12 Months Ended						12 Months Ended							12 Months Ended		0 Months Ended		12 Months Ended	1 Months Ended		0 Months Ended			1 Months Ended				0 Months Ended					36 Months Ended	12 Months Ended				12 Months Ended		1 Months Ended	12 Months Ended										12 Months Ended		93 Months Ended		12 Months Ended
	Dec. 31, 2014 USD ($) t MW	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 EUR (€)	Jun. 12, 2012 USD ($)	Jun. 12, 2012 EUR (€)	Dec. 31, 2014 Minimum [Member]	Dec. 31, 2014 Minimum [Member] Outstanding bank guarantees [Member]	Dec. 31, 2014 Maximum [Member]	Dec. 31, 2014 Maximum [Member] Outstanding bank guarantees [Member]	Dec. 31, 2014 Energy, Raw Materials, and Other Goods and Services [Member] USD ($)	Dec. 31, 2014 State and Local Jurisdiction [Member] USD ($)	Dec. 31, 2014 State and Local Jurisdiction [Member] BRL	Dec. 31, 2014 Machadinho and Barra Grande [Member]	Dec. 31, 2014 Serrado Facao and Estreito Projects [Member]	Feb. 27, 2014 Ligestra [Member] USD ($)	Feb. 27, 2014 Ligestra [Member] EUR (€)	Dec. 31, 2014 Ligestra [Member]	Jan. 31, 2014 Ligestra [Member] Minimum [Member]	Jan. 31, 2014 Ligestra [Member] Maximum [Member]	Feb. 27, 2014 Italian Ministry of Environment and Protection of Land and Sea[Member] USD ($)	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	Dec. 31, 2014 Barra Grande [Member] USD ($) MWh	Dec. 31, 2014 Barra Grande [Member] BRL MWh	Dec. 31, 2013 Barra Grande [Member] USD ($)	Dec. 31, 2013 Barra Grande [Member] BRL	Dec. 31, 2014 Estreito Project [Member] USD ($) MWh	Dec. 31, 2014 Estreito Project [Member] BRL	Oct. 31, 2013 Fusina [Member] USD ($)	Dec. 31, 2009 Fusina [Member] USD ($)	Dec. 31, 2014 Portovesme [Member] USD ($)	Dec. 31, 2009 Portovesme [Member] USD ($)	Dec. 31, 2012 Baie Comeau [Member] USD ($)	Dec. 31, 2012 Mosjoen [Member] USD ($)	Dec. 31, 2014 Machadinho [Member] MWh	Dec. 31, 2014 Serra Do Facao Project [Member] USD ($) MWh	Dec. 31, 2014 Serra Do Facao Project [Member] BRL	Dec. 31, 2013 Serra Do Facao Project [Member] USD ($)	Dec. 31, 2013 Serra Do Facao Project [Member] BRL	Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Sep. 30, 2011 Dampier to Bunbury Natural Gas Pipeline [Member] USD ($)	Sep. 30, 2011 Dampier to Bunbury Natural Gas Pipeline [Member] AUD	Dec. 31, 2014 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] USD ($)	Dec. 31, 2014 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] AUD	Dec. 31, 2013 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] USD ($)	Dec. 31, 2013 Dampier to Bunbury Natural Gas Pipeline [Member] Three-Year Equity Call Plan [Member] AUD	Dec. 31, 2014 Dampier to Bunbury Natural Gas Pipeline [Member] New Equity Call Plan [Member] USD ($)	Dec. 31, 2014 Dampier to Bunbury Natural Gas Pipeline [Member] New Equity Call Plan [Member] AUD
Loss Contingencies [Line Items]
Increase in remediation reserve																																								$ 0	$ 12,000,000	$ 3,000,000	$ 3,000,000	$ 25,000,000	$ 20,000,000
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	277,000,000			228,000,000								49,000,000	130,000,000
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%	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
Commitment to taking share of output in associated projects at cost, in years														30 years	26 years
Investment percentage																																						25.49%	25.49%							30.99%					20.00%	20.00%
Assured power from installed capacity																																		160	160			150	150							120	65	65
Investment percentage under equity method	40.00%								50.00%																									42.18%	42.18%												34.97%	34.97%
Total investment in project																																		132,000,000	355,000,000	143,000,000	336,000,000										66,000,000	178,000,000	82,000,000	192,000,000
Estimated project costs																																						1,930,000,000	5,170,000,000								375,000,000	1,000,000,000
Aluminio's share of project																																						490,000,000	1,320,000,000								130,000,000	350,000,000
Commitment expended on the project																																						475,000,000	1,270,000,000								130,000,000	350,000,000
Percentage of power self-sufficiency upon completion of project with full operating capacity	70.00%
Total energy demand	690
Temporarily declined in energy demand	550
Capacity curtailments of combined smelters	290,000
Additional contributions																																																			17,000,000	24,000,000	141,000,000	176,000,000	10,000,000	11,000,000	12,000,000	12,000,000	1,000,000	1,000,000
Required plan contributions																																																							40,000,000	40,000,000			30,000,000	36,000,000
Prepayments made under the agreement for future gas transmission services																																																							295,000,000	360,000,000
Alcoa's maximum exposure to loss in projects					369,000,000	303,000,000																																																	435,000,000	530,000,000
Purchase obligations due in 2015	137,000,000										3,951,000,000
Purchase obligations due in 2016	142,000,000										2,013,000,000
Purchase obligations due in 2017	145,000,000										1,903,000,000
Purchase obligations due in 2018	147,000,000										1,802,000,000
Purchase obligations due in 2019	147,000,000										1,698,000,000
Purchase obligations due thereafter	1,946,000,000										11,063,000,000
Purchase obligations expenditures	178,000,000	163,000,000	161,000,000
Expiration date of unconditional purchase obligations for energy							2015		2036
Operating leases, expense	227,000,000	232,000,000	244,000,000
Long-term operating leases, minimum annual rentals due in 2015	205,000,000
Long-term operating leases, minimum annual rentals due in 2016	172,000,000
Long-term operating leases, minimum annual rentals due in 2017	131,000,000
Long-term operating leases, minimum annual rentals due in 2018	101,000,000
Long-term operating leases, minimum annual rentals due in 2019	79,000,000
Long-term operating leases, minimum annual rentals due thereafter	165,000,000
Guarantees of third party related to project financing	596,000,000
Guarantees, expiration date							2017	2015	2024	2022
Guarantees, total amount committed	394,000,000
Letters of credit, total amount committed	354,000,000
Letters of credit, expiration date	2015
Total amount committed under outstanding surety bonds	$ 203,000,000
Surety bonds, expiration date	2015