Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 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	988,000,000	1,135,000,000
Fair value measurement, Assets, Sales	(3,000,000)	(10,000,000)
Fair value measurement, Assets, Cost of goods sold	(30,000,000)	(61,000,000)
Fair value measurement, Assets, Other income, net	(4,000,000)	(8,000,000)
Fair value measurement, Assets, Other comprehensive income	(215,000,000)	(336,000,000)
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	9,000,000
Fair value measurement, Assets, Other		25,000,000
Fair value measurement, Assets, Ending balance	745,000,000	745,000,000
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Assets, Other income , net	(4,000,000)	(8,000,000)
Fair value measurement, Liabilities, Beginning balance	161,000,000	169,000,000
Fair value measurement, Liabilities, Sales	(3,000,000)	(5,000,000)
Fair value measurement, Liabilities, Other comprehensive income	38,000,000	32,000,000
Fair value measurement, Liabilities, Purchases, sales, issuances, and settlements	32,000,000	32,000,000
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Ending balance	228,000,000	228,000,000
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, 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	33,000,000	35,000,000
Fair value measurement, Liabilities, Cost of goods sold	(2,000,000)	(3,000,000)
Fair value measurement, Liabilities, Other income, net	2,000,000	1,000,000
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Ending balance	33,000,000	33,000,000
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	1,000,000
Energy Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	6,000,000	2,000,000
Fair value measurement, Assets, Other income, net		2,000,000
Fair value measurement, Assets, Other comprehensive income	37,000,000	39,000,000
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	6,000,000
Fair value measurement, Assets, Other		6,000,000
Fair value measurement, Assets, Ending balance	49,000,000	49,000,000
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Assets, Other income , net		2,000,000
Fair value measurement, Liabilities, Beginning balance	11,000,000	4,000,000
Fair value measurement, Liabilities, Other income, net		(1,000,000)
Fair value measurement, Liabilities, Other comprehensive income	(8,000,000)
Fair value measurement, Liabilities, Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Other	6,000,000
Fair value measurement, Liabilities, Other		6,000,000
Fair value measurement, Liabilities, Ending balance	9,000,000	9,000,000
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)