Debt (Credit Facilities) - Additional Information (Detail) BRL in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2015 BRL	Dec. 31, 2014 USD ($)	Dec. 31, 2014 BRL	Dec. 31, 2013 USD ($)	Jul. 25, 2014 USD ($)
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings						$ 4,000,000,000
Limit of requested increases in lender commitments	$ 990,000,000					$ 500,000,000
Letters of credit sublimit under credit facility	$ 1,000,000,000
Debt instrument, maturity date	Jul. 25, 2019	Jul. 25, 2019
Extension of maturity period	1 year	1 year
Debt instrument, extended maturity date	Jul. 25, 2020	Jul. 25, 2020
Agreement termination date	Jul. 25, 2014	Jul. 25, 2014
Line of credit facility, borrowing	$ 1,890,000,000		$ 1,640,000,000
Line of credit facility, repayment	$ 15,000,000	BRL 48	$ 20,000,000	BRL 47
Weighted-average interest rate	8.49%	8.49%	8.49%	8.49%
Weighted-average maturity days	69 days	69 days	67 days	67 days	213 days
Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings	$ 0		$ 0
Amounts borrowed under the credit facility	0		0
Alcoa [Member]
Debt Instrument [Line Items]
Line of credit facility, repayment	$ 1,890,000,000		$ 1,640,000,000
Weighted-average interest rate	1.61%	1.61%	1.54%	1.54%	1.57%
Minimum [Member]
Debt Instrument [Line Items]
Applicable margin on LIBOR loans	0.50%	0.50%
Maximum [Member]
Debt Instrument [Line Items]
Applicable margin on LIBOR loans	1.50%	1.50%
Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Revolving credit facility, term	5 years	5 years
Debt instrument, maturity date	Jul. 25, 2017	Jul. 25, 2017
Amounts borrowed under the credit facility			$ 0		$ 0
Commercial Paper [Member]
Debt Instrument [Line Items]
Fees paid to maintain credit facility	0.25%	0.25%
Outstanding borrowings	$ 3,750,000,000
Expected Maturity Date Year 2016 [Member]
Debt Instrument [Line Items]
Limit of requested increases in lender commitments	$ 890,000,000
Agreement termination date	Dec. 31, 2016	Dec. 31, 2016
Expected Maturity Date Year 2017 [Member]
Debt Instrument [Line Items]
Limit of requested increases in lender commitments	$ 100,000,000
Agreement termination date	Dec. 31, 2017	Dec. 31, 2017