Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 31, 2016 USD ($)	Sep. 30, 2016 USD ($) Derivative	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) MWh Derivative Agency t	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) MWh t
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other derivative contracts estimated term of quoted market prices, in years				10 years
Number of credit rating agencies | Agency				3
Decrease in unrealized gain		$ 4,000,000	$ (16,000,000)	$ (5,000,000)	$ (19,000,000)
Other Comprehensive Income (Loss) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in unrealized gain	$ 83,000,000
Other Noncurrent Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decreased derivative asset	83,000,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivatives recorded as assets		11,000,000		11,000,000		$ 8,000,000
Fair value of derivatives recorded as liabilities		9,000,000		9,000,000		$ 58,000,000
Net gain (loss) of derivative instruments		1,000,000	7,000,000	4,000,000	34,000,000
Level 1 [Member] | Other Comprehensive Income (Loss) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		$ 1,000,000	0	$ 1,000,000	0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		10		10
Derivatives Not Designated as Hedging Instruments [Member] | Other Expenses (Income), Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		$ 1,000,000	(17,000,000)	$ (8,000,000)	(19,000,000)
Embedded Aluminum Derivative [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized an unrealized gain (loss)		462,000,000	301,000,000	830,000,000	819,000,000
Realized gain (loss) on derivatives		$ 1,000,000	(1,000,000)	$ (4,000,000)	(25,000,000)
Embedded Aluminum Derivative [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		8		8
Embedded Aluminum Derivative [Member] | Level 3 [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		9		9
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		$ (7,000,000)	(4,000,000)	$ (15,000,000)	(5,000,000)
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		3		3
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aluminum forecast sales | t				3,521		3,307
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of gain (loss) expected to be recognized into earnings over the next 12 months				$ 23,000,000
Derivative instruments ineffectiveness		$ 0	0	0	0
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member] | Other Expenses (Income), Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other expenses (income), net		0	1,000,000	1,000,000	2,000,000
Energy Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in unrealized gain	$ 84,000,000
Recognized an unrealized gain (loss)		$ 49,000,000	3,000,000	$ 88,000,000	(8,000,000)
Energy Contracts [Member] | Level 3 [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		3		3
Energy Contracts [Member] | Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments ineffectiveness		$ 0	0	$ 3,000,000	0
Forecasted energy purchases in megawatt hours | MWh				2,130,122		59,409,328
Embedded Credit Derivative [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		5		5
Embedded Credit Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments		$ 8,000,000	$ (13,000,000)	$ 7,000,000	$ (14,000,000)
Embedded Credit Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative		0		0