Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Embedded Aluminum Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	$ 94	$ 497
Fair value measurement, Assets, Other comprehensive income (loss)	(114)	(547)
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	(1)	(1)
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Liabilities, Beginning balance	326	232
Fair value measurement, Liabilities, Other comprehensive income (loss)	388	496
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	(4)	(11)
Fair value measurement, Liabilities, Ending balance	708	708
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	7	15
Embedded Aluminum Derivative [Member] | Sales [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	20	50
Fair value measurement, Liabilities	(9)	(24)
Embedded Aluminum Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	1	1
Fair value measurement, Liabilities		15
Embedded Aluminum Derivative [Member] | Other Expense [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	1
Fair value measurement, Liabilities	7
Embedded Credit Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	33	35
Fair value measurement, Liabilities, Cost of goods sold	(2)	(4)
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	28	28
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(3)	(3)
Embedded Credit Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities		(3)
Embedded Credit Derivative [Member] | Other Expense [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	(3)
Financial Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	254	17
Fair value measurement, Assets, Other comprehensive income (loss)	(7)	100
Fair value measurement, Assets, Purchases, sales, issuances, and settlements		119
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	5	13
Fair value measurement, Assets, Ending balance	222	222
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Financial Contracts [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(17)	(9)
Financial Contracts [Member] | Cost of Goods Sold [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(13)	$ (18)
Financial Contracts [Member] | Other Expense [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	$ (17)