Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	$ 209	$ 268
Fair value liability derivatives	441	394
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	69	99
Fair value liability derivatives	425	376
Derivatives Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	29	24
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	32	73
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	8	2
Derivatives Designated as Hedging Instruments [Member] | Other Current Liabilities [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	22	24
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Liabilities and Deferred Credits [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	403	352
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	140	169
Fair value liability derivatives	16	18
Derivatives Not Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	94	98
Derivatives Not Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	46	71
Derivatives Not Designated as Hedging Instruments [Member] | Other Current Liabilities [Member] | Embedded Credit Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	2	2
Derivatives Not Designated as Hedging Instruments [Member] | Other Noncurrent Liabilities and Deferred Credits [Member] | Embedded Credit Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	$ 14	$ 16