Contingencies and Commitments - Additional Information (Detail) In Millions, unless otherwise specified	1 Months Ended			3 Months Ended			9 Months Ended				1 Months Ended	3 Months Ended			3 Months Ended						3 Months Ended	0 Months Ended	1 Months Ended	3 Months Ended		9 Months Ended				1 Months Ended
	Jul. 31, 2012 Plant	Jun. 30, 2012 USD ($) Installment	Jun. 30, 2012 EUR (€)	Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Sep. 30, 2012 USD ($)	Sep. 30, 2013 USD ($)	Sep. 30, 2012 USD ($)	Sep. 30, 2013 EUR (€)	Jun. 30, 2010 t	Nov. 30, 2006 Curtis v. Alcoa Inc. [Member] Person	Jun. 30, 2013 Alcoa [Member]	Sep. 30, 2013 Alcoa [Member] USD ($)	Sep. 30, 2013 Alcoa [Member] EUR (€)	Jun. 30, 2013 Alumina [Member]	Feb. 21, 2013 Alcoa Trasformazioni [Member] USD ($)	Feb. 21, 2013 Alcoa Trasformazioni [Member] EUR (€)	Feb. 21, 2013 Minimum [Member] USD ($)	Feb. 21, 2013 Maximum [Member] USD ($)	Feb. 21, 2013 Maximum [Member] EUR (€)	Jun. 30, 2013 Aluminium Bahrain [Member] USD ($)	Oct. 09, 2012 Alba [Member] USD ($) Installment	Feb. 27, 2008 Alba [Member] USD ($)	Sep. 30, 2012 Alba [Member] USD ($)	Jun. 30, 2012 Alba [Member] USD ($)	Sep. 30, 2013 Alba [Member]	Oct. 09, 2013 Alba [Member] Subsequent Event [Member] USD ($)	Sep. 30, 2013 Alba [Member] Minimum [Member] USD ($)	Sep. 30, 2013 Alba [Member] Maximum [Member] USD ($)	Nov. 19, 2009 Italian Government [Member] USD ($) Plant	Nov. 19, 2009 Italian Government [Member] EUR (€) Plant	Jul. 31, 2006 Italian Government [Member]
Loss Contingencies [Line Items]
Period of alleged conspiracy, years																							15 years
Damages claimed, value																							$ 1,000
Contingency settlement agreement amount, total																						85
Litigation settlement number of installments																						2
Contingency settlement agreement amount paid																						42.5					42.5
Contingency settlement agreement future payable date																										October 9, 2013
Cash payment for suit settlement																					103			40
Restructuring and other charges, after-tax and noncontrolling interests				108		2	283	19													62			15	18
Restructuring and other charges				151		2	402	27													103				45
Percentage of equity interest																										40.00%
Estimated contingency settlement changes, net of tax																												25	30
Estimated additional reasonably possible charge, minimum		211	159																		0
Estimated additional reasonably possible charge, maximum		403	303																		200
Amount of settlement offer rejected					60
Charges recorded of settlement amount					0
Percentage of allocated cost of settlement												85.00%			15.00%
Cash payment for suit settlement																					25
Retired former employees involved in class action											13,000
Trial length, number of days							8 days
Management estimate for maximum exposure from class action				110			110		85							97	76
Provision for contract losses																		0	97	76
Number of years operating under a power supply structure																																10 years
Charge related to European Commission announcement																														250	173
Number of smelters	2																													2	2
Write-off of receivable																														20	14
Establishment reserve																														230	159
Recovery amount		375	303
Reduction in recovery amount		65	53
Payments to Italian Government		310	250
Payments to Italian Government, installments amount		67	50
Number of installments, litigation payment		5	5
Litigation reserve		211	159										0
Noncurrent asset													$ 55	€ 41
Amount of curtailment, metric-tons-per-year										44,000