Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives	$ 137	$ 153
Fair value liability derivatives	615	289
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives	137	153
Fair value liability derivatives	594	264
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Financial Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives	69	70
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Financial Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives	68	42
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives		41
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	59	46
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	535	218
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	21	25
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives		5
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Credit Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	4	4
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Credit Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	$ 17	$ 16