Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Purchases, sales, issuances, and settlements	$ 0	$ 0
Embedded Aluminum Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	745,000,000	1,135,000,000
Fair value measurement, Assets, Other comprehensive income	414,000,000	(750,000,000)
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	14,000,000	39,000,000
Fair value measurement, Assets, Ending balance	310,000,000	310,000,000
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Liabilities, Beginning balance	228,000,000	169,000,000
Fair value measurement, Liabilities, Other comprehensive (loss) income	48,000,000	80,000,000
Fair value measurement, Liabilities, Purchases, sales, issuances, and settlements		32,000,000
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Other	(3,000,000)	(3,000,000)
Fair value measurement, Liabilities, Ending balance	272,000,000	272,000,000
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	2,000,000	2,000,000
Embedded Aluminum Derivative [Member] | Sales [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	1,000,000	(9,000,000)
Fair value measurement, Assets	1,000,000	(9,000,000)
Fair value measurement, Liabilities	(3,000,000)	(8,000,000)
Embedded Aluminum Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(5,000,000)	(13,000,000)
Fair value measurement, Assets	(5,000,000)	(13,000,000)
Fair value measurement, Liabilities	2,000,000	2,000,000
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	(31,000,000)	(92,000,000)
Fair value measurement, Assets	(31,000,000)	(92,000,000)
Embedded Credit Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	33,000,000	35,000,000
Fair value measurement, Liabilities, Cost of goods sold	(1,000,000)	(4,000,000)
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Ending balance	24,000,000	24,000,000
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(8,000,000)	(7,000,000)
Embedded Credit Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	(8,000,000)	(7,000,000)
Energy Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	49,000,000	2,000,000
Fair value measurement, Assets, Other comprehensive income	(48,000,000)	87,000,000
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	(5,000,000)	1,000,000
Fair value measurement, Assets, Ending balance	7,000,000	7,000,000
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Liabilities, Beginning balance	9,000,000	4,000,000
Fair value measurement, Liabilities, Other comprehensive (loss) income	(1,000,000)	(1,000,000)
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Other	(7,000,000)	(1,000,000)
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(1,000,000)	(2,000,000)
Energy Contracts [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(85,000,000)	(83,000,000)
Fair value measurement, Assets	(85,000,000)	(83,000,000)
Fair value measurement, Liabilities	$ (1,000,000)	$ (2,000,000)