Derivatives and Other Financial Instruments - Schedule of Fair Values and Corresponding Level of Fair Value Hierarchy of Outstanding Derivative Contracts Recorded as Assets (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	$ 346	$ 390	$ 596
Level 1 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	11	6
Level 3 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	305	355
Level 2 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	30	32
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	89	69
Derivatives Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Aluminum Contracts [Member] | Level 1 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	5	4
Derivatives Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Aluminum Contracts [Member] | Level 3 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	7	9
Derivatives Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Foreign Exchange Contracts [Member] | Level 1 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	6	2
Derivatives Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Interest Rate Contracts [Member] | Level 2 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	9	9
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Aluminum Contracts [Member] | Level 3 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	17	16
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Interest Rate Contracts [Member] | Level 2 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	21	23
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Energy Contracts [Member] | Level 3 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	24	6
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	257	324
Derivatives Not Designated as Hedging Instruments [Member] | Prepaid Expenses and Other Current Assets [Member] | Aluminum Contracts [Member] | Level 3 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	116	149
Derivatives Not Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Aluminum Contracts [Member] | Level 3 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	141	175
Margin Held [Member] | Prepaid Expenses and Other Current Assets [Member] | Interest Rate Contracts [Member] | Level 2 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives		$ 3