Basis of Presentation - Schedule of Cost Allocation (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basis Of Presentation [Line Items]
Cost of goods sold			$ 10,081	$ 8,991	$ 7,877
Selling, general administrative, and other expenses			248	280	356
Research and development expenses			31	32	33
Provision for depreciation, depletion, and amortization			733	750	718
Restructuring and other charges	$ 138	$ 297	527	309	318
Interest expense			122	104	243
Other income, net			$ (64)	$ (27)	65
Alcoa Corporation [Member]
Basis Of Presentation [Line Items]
Cost of goods sold					40
Selling, general administrative, and other expenses					150
Research and development expenses					2
Provision for depreciation, depletion, and amortization					18
Restructuring and other charges					1
Interest expense					198
Other income, net					$ (7)