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	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	$ 371	$ 390	$ 596
Level 1 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	7	6
Level 3 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	337	355
Level 2 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	31	32
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	83	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	2	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	10	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	5	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	6	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	24	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	25	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	11	6
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	292	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	133	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	159	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	$ 4	$ 3