Derivatives and Other Financial Instruments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) Derivative	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) MWh Derivative Smelter Refinery kt	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) MWh kt	Dec. 31, 2016 USD ($) MWh
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
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 June 30, 2018 and December 31, 2017, this financial contract hedges forecasted electricity purchases of 7,586,964 and 8,805,456, respectively, megawatt hours.
Recognized an unrealized gain (loss)	$ 12,000,000		$ 34,000,000
Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets	7,000,000		7,000,000		$ 44,000,000
Fair value of derivative contracts recorded as liabilities	67,000,000		67,000,000		$ 117,000,000
Other Expenses (Income) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized gain (loss)		$ (2,000,000)		$ (24,000,000)
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	(55,000,000)	25,000,000	13,000,000	(25,000,000)
Realized gain (loss) on derivatives	$ (6,000,000)	(4,000,000)	$ (7,000,000)	(5,000,000)
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of smelters | Smelter			9
Number of refineries | Refinery			3
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]
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]
Number of derivative instruments | Derivative	1		1
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)	(29,000,000)	$ 11,000,000	(4,000,000)
Energy Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in derivative asset						$ (84,000,000)
Energy Contracts [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other income						7,000,000
Cost of goods sold						$ (6,000,000)
Energy Contracts [Member] | Derivative D11 [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cost of goods sold			(41,000,000)
Energy Contracts [Member] | Other Comprehensive Loss [Member] | Derivative D11 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized	$ (24,000,000)	(62,000,000)	$ (75,000,000)	107,000,000
Energy Contracts [Member] | Derivatives Designated as Hedging Instruments [Member] | Derivative D11 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forecasted energy purchases in megawatt hours | MWh			7,586,964		8,805,456	1,969,544
Energy 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		(30,000,000)	$ 34,000,000	4,000,000
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			2,693		2,859
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]
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		6,000,000	$ 6,000,000	(8,000,000)
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	1		1
Embedded Credit Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative contract period			10 years
Derivative contract period, description			As Alcoa Corporation does not have outstanding 30-year debt, the Company’s estimated 30-year debt yield is represented by the sum of (i) the excess of the yield on Alcoa’s outstanding notes due 2026 over the yield on only the Ba/BB-rated company debt included in Barclays High Yield Index for intermediate (10-year) credits and (ii) the yield on only the Ba/BB-rated company debt included in Barclays High Yield Index for long (30-year) credits. In accordance with the terms of the power contract, this calculation may be changed in January of each calendar year.
Embedded Credit Derivative [Member] | Negotiated multiplier [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative contract period			30 years
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	$ (6,000,000)	$ (5,000,000)	$ 5,000,000	$ (1,000,000)