Derivatives and Other Financial Instruments - Additional Information (Detail)	1 Months Ended	3 Months Ended		3 Months Ended			3 Months Ended										3 Months Ended				12 Months Ended	3 Months Ended					3 Months Ended		12 Months Ended			3 Months Ended		1 Months Ended
	Jul. 31, 2012 Contract Plant	Mar. 31, 2015 Agency	Mar. 31, 2015 Level 3 [Member] Derivative	Mar. 31, 2015 Level 1 [Member] USD ($)	Mar. 31, 2014 Level 1 [Member] USD ($)	Dec. 31, 2014 Level 1 [Member] USD ($)	Mar. 31, 2015 Derivatives Not Designated as Hedging Instruments [Member] Other Income Or Other Expense [Member] USD ($)	Mar. 31, 2014 Derivatives Not Designated as Hedging Instruments [Member] Other Income Or Other Expense [Member] USD ($)	Mar. 31, 2015 Foreign Exchange Contracts Two [Member] USD ($)	Mar. 31, 2015 Foreign Exchange Contracts Two [Member] Forward Contracts [Member] USD ($)	Mar. 31, 2015 Foreign Exchange Contracts Two [Member] Forward Contracts [Member] BRL	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] USD ($)	Mar. 31, 2014 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] USD ($)	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Level 3 [Member] Derivative	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Level 3 [Member] Cash Flow Hedging [Member] Derivative	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Level 3 [Member] Derivatives Not Designated as Hedging Instruments [Member] Derivative	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Derivatives Designated as Hedging Instruments [Member] Cash Flow Hedging [Member] USD ($)	Mar. 31, 2014 Embedded Aluminum Derivative [Member] Derivatives Designated as Hedging Instruments [Member] Cash Flow Hedging [Member] USD ($)	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Derivatives Designated as Hedging Instruments [Member] Cash Flow Hedging [Member] Nonoperating Income (Expense) [Member] USD ($)	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Derivatives Designated as Hedging Instruments [Member] Scenario, Forecast [Member] t	Dec. 31, 2014 Embedded Aluminum Derivative [Member] Derivatives Designated as Hedging Instruments [Member] Scenario, Forecast [Member] t	Mar. 31, 2015 Embedded Aluminum Derivative [Member] Derivatives Not Designated as Hedging Instruments [Member] USD ($)	Mar. 31, 2014 Embedded Aluminum Derivative [Member] Derivatives Not Designated as Hedging Instruments [Member] Maximum [Member] USD ($)	Mar. 31, 2015 Energy Contracts [Member] USD ($)	Mar. 31, 2014 Energy Contracts [Member] USD ($)	Mar. 31, 2015 Energy Contracts [Member] Level 3 [Member] Cash Flow Hedging [Member] Derivative	Mar. 31, 2015 Energy Contracts [Member] Derivatives Designated as Hedging Instruments [Member] USD ($) MWh	Mar. 31, 2014 Energy Contracts [Member] Derivatives Designated as Hedging Instruments [Member] USD ($)	Dec. 31, 2014 Energy Contracts [Member] Derivatives Designated as Hedging Instruments [Member] MWh	Mar. 31, 2015 Embedded Credit Derivative [Member] Level 3 [Member] Derivative	Mar. 31, 2015 Embedded Credit Derivative [Member] Level 3 [Member] Derivatives Not Designated as Hedging Instruments [Member] Derivative	Mar. 31, 2015 Embedded Credit Derivative [Member] Derivatives Not Designated as Hedging Instruments [Member] USD ($)	Mar. 31, 2014 Embedded Credit Derivative [Member] Derivatives Not Designated as Hedging Instruments [Member] USD ($)	Jul. 31, 2012 Power Contract One [Member]	Jul. 31, 2012 Power Contract Two [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian/Brazilian-denominated loan due in 2014										$ 231,000,000	543,000,000
Maturity date of forward contract										Mar. 31, 2014	Mar. 31, 2014
Loss recognized on forward contract									4,000,000
Fair value of derivatives recorded as assets				2,000,000
Fair value of derivatives recorded as liabilities				37,000,000		31,000,000
Net gain (loss) of derivative instruments				26,000,000	(25,000,000)		3,000,000	1,000,000														1,000,000	1,000,000
Number of derivative instruments			9											7	5	2										1				1	0
Other derivative contracts estimated term of quoted market prices, in years		10 years
Number of contracts	2
Power contracts maturity date																																		Jun. 30, 2014	Jun. 30, 2014
Number of smelters	2
Number of credit rating agencies		3
Aluminum forecast sales																				3,566,000	3,610,000
Recognized an unrealized (loss)												(98,000,000)	30,000,000											6,000,000	5,000,000
Unrealized gain loss on derivatives												(12,000,000)	(5,000,000)
Amount of loss expected to be recognized into earnings over the next 12 months																	3,000,000
Other expenses (income), net																			1,000,000
Derivative instruments ineffectiveness																	0	0									0	0
Forecasted energy purchases in megawatt hours																											59,409,328		59,409,328
Derivative gain																																$ 2,000,000	$ 1,000,000