Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands, CAD in Millions, BRL in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2014 Plant Contract	Dec. 31, 2015 USD ($) MWh Derivative Agency t	Dec. 31, 2014 USD ($) t	Dec. 31, 2013 USD ($)	Dec. 31, 2014 BRL	Dec. 31, 2014 CAD
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
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivatives recorded as assets		$ 8,000,000	$ 2,000,000
Fair value of derivatives recorded as liabilities		58,000,000	31,000,000
Net gain (loss) of derivative instruments		19,000,000	(15,000,000)	$ 4,000,000
Net gain of cash flow hedges				9,000,000
Derivatives Not Designated as Hedging Instruments [Member] | Other (Income) Expenses, Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		(25,000,000)	(13,000,000)	36,000,000
Foreign Exchange Contracts One [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			$ 53,000,000			CAD 58
Maturity date of forward contract			Aug. 05, 2014
Foreign Exchange Contracts One [Member] | Forward Contracts [Member] | Canadian Denominated Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (Loss) recognized on forward contract			$ 1,000,000
Foreign Exchange Contracts Two [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (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		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)		1,155,000,000	$ 78,000,000	190,000,000
Realized gain (loss) on derivatives		$ (21,000,000)	$ (28,000,000)	(29,000,000)
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
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
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aluminum forecast sales | t		3,307	3,610
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 gain (loss) expected to be recognized into earnings over the next 12 months		$ 45,000,000
Derivative instruments ineffectiveness		0	$ 0	0
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		$ (8,000,000)	(15,000,000)	28,000,000
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
Energy Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized an unrealized gain (loss)		$ (2,000,000)	(4,000,000)	3,000,000
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
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		$ (3,000,000)	0	0
Forecasted energy purchases in megawatt hours | MWh		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
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		$ (17,000,000)	$ 2,000,000	$ 8,000,000
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
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