Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	$ 794	$ 1,137
Fair value liability derivatives	270	208
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	771	1,068
Fair value liability derivatives	205	173
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	51	72
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	671	994
Derivatives Designated as Hedging Instruments [Member] | Other Noncurrent Assets [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	49	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	14	9
Derivatives Designated as Hedging Instruments [Member] | Other Current Liabilities [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	9	4
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	182	160
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	23	69
Fair value liability derivatives	65	35
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	23	69
Derivatives Not Designated as Hedging Instruments [Member] | Other Current Liabilities [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	8
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	5	6
Derivatives Not 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	24
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	$ 28	$ 29