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, 2016	Sep. 30, 2016
Embedded Aluminum Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	$ 745	$ 1,135
Fair value measurement, Assets, Other comprehensive income (loss)	(414)	(750)
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	14	39
Fair value measurement, Assets, Ending balance	310	310
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Liabilities, Beginning balance	228	169
Fair value measurement, Liabilities, Other comprehensive income (loss)	48	80
Fair value measurement, Liabilities, Purchases, sales, issuances, and settlements		32
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Other	(3)	(3)
Fair value measurement, Liabilities, Ending balance	272	272
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	2	2
Embedded Aluminum Derivative [Member] | Sales [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	1	(9)
Fair value measurement, Assets	1	(9)
Fair value measurement, Liabilities	(3)	(8)
Embedded Aluminum Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(5)	(13)
Fair value measurement, Assets	(5)	(13)
Fair value measurement, Liabilities	2	2
Embedded Aluminum Derivative [Member] | Cost of Goods Sold [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(31)	(92)
Fair value measurement, Assets	(31)	(92)
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	(1)	(4)
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Ending balance	24	24
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(8)	(7)
Embedded Credit Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	(8)	(7)
Energy Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	49	2
Fair value measurement, Assets, Other comprehensive income (loss)	48	87
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	(5)	1
Fair value measurement, Assets, Ending balance	7	7
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Liabilities, Beginning balance	9	4
Fair value measurement, Liabilities, Other comprehensive income (loss)	(1)	(1)
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Other	(7)	(1)
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(1)	(2)
Energy Contracts [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(85)	(83)
Fair value measurement, Assets	(85)	(83)
Fair value measurement, Liabilities	$ (1)	$ (2)