Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands, BRL in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2014 Plant Contract	Sep. 30, 2015 USD ($) Derivative	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) MWh Derivative Agency t	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) MWh t	Sep. 30, 2015 BRL Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other derivative contracts estimated term of quoted market prices, in years				10 years
Number of contracts | Contract	2
Number of smelters | Plant	2
Number of credit rating agencies | Agency				3
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		9		9			9
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivatives recorded as assets		$ 6,000,000		$ 6,000,000		$ 2,000,000
Fair value of derivatives recorded as liabilities		54,000,000		54,000,000		$ 31,000,000
Net gain (loss) of derivative instruments		(7,000,000)	$ (15,000,000)	34,000,000	$ (2,000,000)
Derivatives Not Designated as Hedging Instruments [Member] | Other (income) expenses, net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		(17,000,000)	(21,000,000)	(19,000,000)	(15,000,000)
Foreign Exchange Contracts Two [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss recognized on forward contract				4,000,000
Foreign Exchange Contracts Two [Member] | Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian/Brazilian-denominated loan due in 2014		231,000,000		$ 231,000,000			BRL 543
Maturity date of forward contract				Mar. 31, 2014
Embedded Aluminum Derivative [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized an unrealized gain (loss)		301,000,000	(4,000,000)	$ 819,000,000	(28,000,000)
Realized gain (loss) on derivatives		$ (1,000,000)	(8,000,000)	$ (25,000,000)	(19,000,000)
Embedded Aluminum Derivative [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		7		7			7
Embedded Aluminum Derivative [Member] | Level 3 [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		5		5			5
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		$ (4,000,000)	(11,000,000)	$ (5,000,000)	(12,000,000)
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		2		2			2
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of loss expected to be recognized into earnings over the next 12 months				$ 41,000,000
Derivative instruments ineffectiveness		$ 0		0	0
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member] | Other (income) expenses, net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other expenses (income), net				$ 2,000,000	(1,000,000)
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Maximum [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments ineffectiveness			0
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Maximum [Member] | Cash Flow Hedging [Member] | Other (income) expenses, net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other expenses (income), net		1,000,000	(1,000,000)
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Scenario, Forecast [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aluminum forecast sales | t				3,369		3,610
Energy Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized an unrealized gain (loss)		$ 3,000,000	(28,000,000)	$ (8,000,000)	(10,000,000)
Energy Contracts [Member] | Level 3 [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		1		1			1
Energy Contracts [Member] | Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments ineffectiveness		$ 0	0	$ 0	0
Forecasted energy purchases in megawatt hours | MWh				59,409,328		59,409,328
Embedded Credit Derivative [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		1		1			1
Embedded Credit Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		$ (13,000,000)	$ (10,000,000)	$ (14,000,000)	$ (3,000,000)
Embedded Credit Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		0		0			0
Power Contract One [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power contracts maturity date	Jun. 30, 2014
Power Contract Two [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power contracts maturity date	Jun. 30, 2014