Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Embedded Aluminum Derivative [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance		$ 497
Fair value measurement, Assets, Other comprehensive income	$ 40	(499)
Fair value measurement, Assets, Purchases, sales, issuances, and settlements	0	0
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	1	(2)
Fair value measurement, Assets, Ending balance	41
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Liabilities, Beginning balance	1,146	232
Fair value measurement, Liabilities, Other comprehensive income	(745)	(1,022)
Fair value measurement, Liabilities, Purchases, sales, issuances, and settlements	0	0
Fair value measurement, Liabilities,Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Other	(13)	(16)
Fair value measurement, Liabilities, Ending balance	269	1,146
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(19)	18
Fair value measurement, Liabilities, Other comprehensive income	745	1,022
Financial Contracts [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	197	17
Fair value measurement, Assets, Other comprehensive income	(11)	88
Fair value measurement, Assets, Purchases, sales, issuances, and settlements	0	119
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	(12)	11
Fair value measurement, Assets, Ending balance	112	197
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Embedded Credit Derivative [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	27	35
Fair value measurement, Liabilities, Cost of goods sold	(3)	(5)
Fair value measurement, Liabilities, Purchases, sales, issuances, and settlements	0	0
Fair value measurement, Liabilities,Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Ending balance	20	27
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(4)	(3)
Sales [Member] | Embedded Aluminum Derivative [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	0	3
Fair value measurement, Liabilities	(100)	(110)
Sales [Member] | Financial Contracts [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	0
Cost of Goods Sold [Member] | Financial Contracts [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(62)	(31)
Other Expense [Member] | Embedded Aluminum Derivative [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	0	1
Fair value measurement, Liabilities	(19)	18
Other Expense [Member] | Financial Contracts [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	0	(7)
Other Expense [Member] | Embedded Credit Derivative [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	$ (4)	$ (3)