Derivatives and Other Financial Instruments - Additional Information (Detail) t in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) MWh Derivative Agency t	Mar. 31, 2015 USD ($)	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
5.55% Notes Due February 2017 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term deb, current	$ 750,000,000
Debt instrument, interest rate	5.55%
Debt instrument, maturity date	2017-02
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative	9
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivatives recorded as assets	$ 4,000,000		$ 8,000,000
Fair value of derivatives recorded as liabilities	35,000,000		$ 58,000,000
Net gain (loss) of derivative instruments	2,000,000	$ 26,000,000
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	(3,000,000)	3,000,000
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)	(115,000,000)	(98,000,000)
Realized gain (loss) on derivatives	$ 5,000,000	12,000,000
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	7
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	5
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	3,246		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	$ 46,000,000
Derivative instruments ineffectiveness	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	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)	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	2
Energy Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized an unrealized gain (loss)	$ 6,000,000	6,000,000
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	1
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	$ 3,000,000	0
Forecasted energy purchases in megawatt hours | MWh	59,409,328		59,409,328
Energy Contracts [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	$ (5,000,000)
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	1
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	$ 1,000,000	$ 2,000,000
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