Inventories - Schedule of Effects of Change in Accounting Principle From LIFO to Average Cost (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jan. 01, 2018
Statement of Consolidated Operations
Cost of goods sold	$ 2,180	$ 2,302
Provision for income taxes	150	151
Net (loss) income	(58)	340
Net income attributable to noncontrolling interest	141	145
Net (loss) income attributable to Alcoa Corporation	$ (199)	$ 195
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic	$ (1.07)	$ 1.05
Diluted	$ (1.07)	$ 1.04
Statement of Consolidated Comprehensive Income
Comprehensive (loss) income	$ (318)	$ 959
Comprehensive (loss) income attributable to Alcoa Corporation	(468)	847
Comprehensive income attributable to noncontrolling interest	150	112
Consolidated Balance Sheet
Inventories	1,799		$ 1,819
Prepaid expenses and other current assets	285		320
Retained earnings	371		570
Noncontrolling interest	1,926		1,970
Statement of Consolidated Cash Flows
Net (loss) income	(58)	340
Deferred income taxes	33	2
Decrease (Increase) in inventories (H)	17	(248)
Change in Accounting Principle from LIFO to Average Cost [Member] | As Computed under LIFO [Member]
Statement of Consolidated Operations
Cost of goods sold	2,228
Provision for income taxes	137
Net (loss) income	(93)
Net income attributable to noncontrolling interest	127
Net (loss) income attributable to Alcoa Corporation	$ (220)
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic	$ (1.19)
Diluted	$ (1.19)
Statement of Consolidated Comprehensive Income
Comprehensive (loss) income	$ (353)
Comprehensive (loss) income attributable to Alcoa Corporation	(489)
Comprehensive income attributable to noncontrolling interest	136
Consolidated Balance Sheet
Inventories	1,579
Prepaid expenses and other current assets	276
Retained earnings	121
Noncontrolling interest	1,947
Statement of Consolidated Cash Flows
Net (loss) income	(93)
Deferred income taxes	20
Decrease (Increase) in inventories (H)	65
Change in Accounting Principle from LIFO to Average Cost [Member] | Effect of Change [Member]
Statement of Consolidated Operations
Cost of goods sold	(48)
Provision for income taxes	13
Net (loss) income	35
Net income attributable to noncontrolling interest	14
Net (loss) income attributable to Alcoa Corporation	$ 21
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic	$ 0.12
Diluted	$ 0.12
Statement of Consolidated Comprehensive Income
Comprehensive (loss) income	$ 35
Comprehensive (loss) income attributable to Alcoa Corporation	21
Comprehensive income attributable to noncontrolling interest	14
Consolidated Balance Sheet
Inventories	220
Prepaid expenses and other current assets	9
Retained earnings	250
Noncontrolling interest	(21)
Statement of Consolidated Cash Flows
Net (loss) income	35
Deferred income taxes	13
Decrease (Increase) in inventories (H)	$ (48)
Change in Accounting Principle from LIFO to Average Cost [Member] | As Originally Reported [Member]
Statement of Consolidated Operations
Cost of goods sold		2,381
Provision for income taxes		138
Net (loss) income		274
Net income attributable to noncontrolling interest		124
Net (loss) income attributable to Alcoa Corporation		$ 150
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic		$ 0.81
Diluted		$ 0.80
Statement of Consolidated Comprehensive Income
Comprehensive (loss) income		$ 893
Comprehensive (loss) income attributable to Alcoa Corporation		802
Comprehensive income attributable to noncontrolling interest		91
Consolidated Balance Sheet
Inventories			1,644
Prepaid expenses and other current assets			301
Retained earnings			341
Noncontrolling interest			2,005
Statement of Consolidated Cash Flows
Net (loss) income		274
Deferred income taxes		(11)
Decrease (Increase) in inventories (H)		(169)
Change in Accounting Principle from LIFO to Average Cost [Member] | Effect of Change [Member]
Statement of Consolidated Operations
Cost of goods sold		(79)
Provision for income taxes		13
Net (loss) income		66
Net income attributable to noncontrolling interest		21
Net (loss) income attributable to Alcoa Corporation		$ 45
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic		$ 0.24
Diluted		$ 0.24
Statement of Consolidated Comprehensive Income
Comprehensive (loss) income		$ 66
Comprehensive (loss) income attributable to Alcoa Corporation		45
Comprehensive income attributable to noncontrolling interest		21
Consolidated Balance Sheet
Inventories			175
Prepaid expenses and other current assets			19
Retained earnings			229	$ 205
Noncontrolling interest			$ (35)	$ (35)
Statement of Consolidated Cash Flows
Net (loss) income		66
Deferred income taxes		13
Decrease (Increase) in inventories (H)		$ (79)