Derivatives and Other Financial Instruments - Schedule of Net Fair Values of Level 3 Derivative Instruments and Effect of Hypothetical Change (Increase or Decrease of 10%) in Market Prices or Rates (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Embedded Aluminum Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset/(liability)	$ (228)
Index change of + / -10%	343
Embedded Credit Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset/(liability)	(20)
Index change of + / -10%	2
Financial Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset/(liability)	112
Index change of + / -10%	$ 35