Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2016 USD ($) Derivative	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) MWh Derivative Agency t	Mar. 31, 2016	Jun. 30, 2015 USD ($)	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
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative	10		10
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivatives recorded as assets	$ 6,000,000		$ 6,000,000			$ 8,000,000
Fair value of derivatives recorded as liabilities	11,000,000		11,000,000			$ 58,000,000
Net gain (loss) of derivative instruments	1,000,000	$ 15,000,000	3,000,000		$ 41,000,000
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	(6,000,000)	(5,000,000)	(9,000,000)		(2,000,000)
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)	(253,000,000)	616,000,000	(368,000,000)		518,000,000
Realized gain (loss) on derivatives	$ 1,000,000	(12,000,000)	$ 5,000,000		(24,000,000)
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 Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aluminum forecast sales | t			3,261			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			$ 25,000,000
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		1,000,000
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	$ (4,000,000)	(2,000,000)	$ (8,000,000)		(1,000,000)
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
Energy Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized an unrealized gain (loss)	$ 45,000,000	(17,000,000)	$ 39,000,000		(11,000,000)
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			59,409,328			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	$ (2,000,000)	$ (3,000,000)	$ (1,000,000)		$ (1,000,000)
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