Derivatives and Other Financial Instruments - Additional Information (Detail) kT in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) Derivative	Sep. 30, 2016 USD ($) Derivative	Sep. 30, 2017 USD ($) MWh Derivative Refinery Smelter kT	Sep. 30, 2016 USD ($) Derivative	Dec. 31, 2016 USD ($) MWh kT
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets	$ 222,000,000		$ 222,000,000		$ 514,000,000
Fair value of derivative contracts recorded as liabilities	736,000,000		$ 736,000,000		267,000,000
Other derivative contracts estimated term of quoted market prices, in years			10 years
Decrease in unrealized gain	(6,000,000)	$ 4,000,000	$ (22,000,000)	$ (5,000,000)
Recognized an unrealized gain (loss)	12,000,000		$ 12,000,000
Electricity purchases			In addition, in January 2017, Alcoa Corporation entered into a new financial contract that hedges the anticipated power requirements at this smelter for the period from August 2017 through July 2021 (see D11 above). At September 30, 2017, this financial contract hedges forecasted electricity purchases of 9,424,800 megawatt hours.
Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets	43,000,000		$ 43,000,000		5,000,000
Fair value of derivative contracts recorded as liabilities	92,000,000		92,000,000		2,000,000
Other Comprehensive Loss [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized	(7,000,000)		100,000,000
Other Income [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in unrealized gain					7,000,000
Cost of Goods Sold [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in unrealized gain					6,000,000
Realized gain (loss)			65,000,000
Other Expenses (Income) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized	1,000,000		(23,000,000)
Realized gain (loss)	(6,000,000)		(2,000,000)
Cash Flow Hedging [Member] | Other Comprehensive Loss [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized	$ (39,000,000)		$ (64,000,000)
London Metal Exchange [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative	2		2
LME Plus Midwest Premium [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative	3		3
Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets	$ 222,000,000		$ 222,000,000		497,000,000
Fair value of derivative contracts recorded as liabilities	$ 674,000,000		$ 674,000,000		204,000,000
Number of derivative instruments | Derivative	1		1
Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets					17,000,000
Fair value of derivative contracts recorded as liabilities	$ 62,000,000		$ 62,000,000		$ 63,000,000
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	(15,000,000)	1,000,000	(19,000,000)	(8,000,000)
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)	(502,000,000)	(462,000,000)	(1,043,000,000)	(830,000,000)
Realized gain (loss) on derivatives	(29,000,000)	4,000,000	$ (74,000,000)	(1,000,000)
Embedded Aluminum Derivative [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of smelters | Smelter			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 | kT			2,926		3,127
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			$ (61,000,000)
Derivative instruments ineffectiveness			0	0
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)	$ (7,000,000)	$ (15,000,000)	$ (15,000,000)
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	3	2	3
Energy Contracts [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of refineries | Refinery			3
Financial Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in derivative asset					$ (84,000,000)
Recognized an unrealized gain (loss)	$ 1,000,000	$ 49,000,000	$ 6,000,000	$ 88,000,000
Financial 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		3,000,000
Forecasted energy purchases in megawatt hours | MWh			9,424,800		1,969,544
Financial Contracts [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	(11,000,000)		$ (7,000,000)
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	$ 3,000,000	$ 8,000,000	$ 3,000,000	$ 7,000,000
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	1		1