Income Taxes - Additional Information (Detail) € in Millions, $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 EUR (€)	Dec. 31, 2018 USD ($) Filer	Dec. 31, 2018 EUR (€) Filer	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income Taxes [Line Items]
Corporate income tax rate			21.00%	21.00%	35.00%	35.00%
Increase (decrease) in deferred tax asset due to tax holiday			$ 5		$ (26)
Deferred income taxes			301
Deferred tax assets			2,556		3,112
Deferred tax assets, valuation allowance			1,684		$ 1,927	$ 1,755	$ 712
Foreign undistributed net earnings for which no deferred taxes have been provided			1,010
Earnings generated during 2018 and 2017 deemed to be permanently reinvested			$ 230
Percentage of the effect of unrecognized tax benefit, if recorded			2.00%	2.00%	1.00%	10.00%
Interest and penalties recognized			$ 10		$ 1	$ 1
Income related to accrued interest and penalties			1		6	$ 2
Amount accrued for payment of interest and penalties			$ 12		$ 2
Net operating losses carry forward period					20 years
Net operating losses carryback period					2 years
Percentage of taxable income related to net operating losses carry forward and carryback period			80.00%	80.00%	100.00%
Discrete income tax charge resulting from preliminary analysis of provisions of TCJA					$ 22
Change in deferred tax assets related to TCJA					(506)
Change in valuation allowance related to TCJA					(433)
Change in deferred tax liabilities related to TCJA					$ (51)
Threshold percentage related to Tax Cuts and Jobs Act of 2017					3.00%
Elysis Limited Partnership [Member]
Income Taxes [Line Items]
Deferred tax assets, valuation allowance			$ 86
Initial cash investment			$ 5
Minimum [Member]
Income Taxes [Line Items]
One-time deemed repatriation of foreign undistributed net earnings, percentage					8.00%
Income tax rate on global intangible low tax income (GILTI)					10.50%
Tax rate on base erosion payments			5.00%	5.00%
Percentage of deductible expenses					3.00%
Maximum [Member]
Income Taxes [Line Items]
One-time deemed repatriation of foreign undistributed net earnings, percentage					15.50%
Income tax rate on global intangible low tax income (GILTI)					21.00%
Tax rate on base erosion payments			10.00%	10.00%
Iceland [Member]
Income Taxes [Line Items]
Deferred tax assets, valuation allowance					$ 94
Discrete income tax charge charge					60
Tax charge to Accumulated other comprehensive loss					$ 34
Iceland [Member] | Minimum [Member]
Income Taxes [Line Items]
Deferred tax assets related to tax loss carryforwards expiration year					2017
Iceland [Member] | Maximum [Member]
Income Taxes [Line Items]
Deferred tax assets related to tax loss carryforwards expiration year					2026
Secretariat of the Federal Revenue Bureau of Brazil [Member]
Income Taxes [Line Items]
Tax credit carryforward, limitations on use			30.00%	30.00%
Tax Authority, Spain [Member]
Income Taxes [Line Items]
Tax credit carryforward, limitations on use			25.00%	25.00%
Charge recorded in provision for income taxes to establish liability for estimated loss	$ 30	€ 26	$ 30	€ 26
Interest recorded in provision for income taxes to establish liability for estimated loss			$ 10	€ 9
Percentage of share of the estimated loss	49.00%	49.00%	49.00%	49.00%
Foreign [Member]
Income Taxes [Line Items]
Deferred income taxes			$ 291
Percentage of net deferred tax asset relates to four of Alcoa Corporation’s income tax filers			80.00%	80.00%
Number of Alcoa Corporation’s income tax filers | Filer			4	4
Deferred tax assets			$ 1,665
Deferred tax assets, valuation allowance			905
Foreign [Member] | AofA [Member]
Income Taxes [Line Items]
Deferred tax assets			$ 220
State and Local Jurisdiction [Member] | Earliest Tax Year [Member]
Income Taxes [Line Items]
Income tax return, year under examination			2006	2006
State and Local Jurisdiction [Member] | Latest Tax Year [Member]
Income Taxes [Line Items]
Income tax return, year under examination			2017	2017
Alcoa World Alumina Brasil [Member]
Income Taxes [Line Items]
Corporate income tax rate					15.25%	34.00%
Tax holiday period					10 years
Increase (decrease) in deferred tax asset due to tax holiday			$ 5		$ (26)
Increase (decrease) in discrete income tax charge and benefit due tax holiday			5		(26)
Increase (decrease) in deferred tax asset due to tax holiday after noncontrolling interest			3		(15)
Increase (decrease) in discrete income tax charge and benefit due to tax holiday after noncontrolling interest			3		$ (15)
Alcoa World Alumina Brasil [Member] | Foreign [Member]
Income Taxes [Line Items]
Deferred tax assets			140
Alcoa Aluminio [Member] | Foreign [Member]
Income Taxes [Line Items]
Deferred tax assets			209
Espanola [Member] | Foreign [Member]
Income Taxes [Line Items]
Deferred tax assets			$ 94