Accumulated Other Comprehensive (Loss) Income - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Pension and other postretirement benefits (L)
Total Other comprehensive income (loss)	$ (7)		$ 1	$ (23)	$ (27)
Acquisition of noncontrolling interest (C)			(288)
Foreign currency translation
Other comprehensive income (loss)	87		109	489	(143)
Joint venture formation (C)		$ (76)
Cash flow hedges (M)
Net change from periodic revaluations	(207)		(143)	(420)	(179)
Net amount reclassified to earnings	90		74	265	201
Total Accumulated other comprehensive (loss) income	(4,770)			(4,770)		$ (5,110)
Alcoa Corporation [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	5	(11)	22	(11)
Unrecognized net actuarial gain/loss and prior service cost/benefit	1		(5)	1	9
Tax benefit	1		1	3	(2)
Total Other comprehensive income (loss) before reclassifications, net of tax	2		(4)	4	7
Amortization of net actuarial gain/loss and prior service cost/benefit	5		5	19	16
Total amount reclassified from Accumulated other comprehensive loss, net of tax	5		5	19	16
Total Other comprehensive income (loss)	7		1	23	23
Balance at end of period	12		12	12	12
Foreign currency translation
Balance at beginning of period	(3,833)	(4,194)	(2,775)	(4,194)	(2,593)
Other comprehensive income (loss)	87		144	479	(38)
Joint venture formation (C)				(31)
Balance at end of period	(3,746)		(3,719)	(3,746)	(3,719)
Cash flow hedges (M)
Balance at beginning of period	(936)	$ (905)	(984)	(905)	(1,052)
Net change from periodic revaluations	(207)		(143)	(420)	(179)
Tax benefit	34		33	75	33
Total Other comprehensive loss before reclassifications, net of tax	(173)		(110)	(345)	(146)
Net amount reclassified to earnings	90		74	265	201
Tax expense	(17)		(15)	(51)	(38)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	73		59	214	163
Total Other comprehensive (loss) income	(100)		(51)	(131)	17
Balance at end of period	(1,036)		(1,035)	(1,036)	(1,035)
Total Accumulated other comprehensive (loss) income	(4,770)		(4,742)	(4,770)	(4,742)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	86		73	257	205
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	4			6
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings					(1)
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges (M)
Net amount reclassified to earnings			1	2	(3)
Non-controlling Interest [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period			(9)		(15)
Unrecognized net actuarial gain/loss and prior service cost/benefit			(2)		5
Tax benefit					(2)
Total Other comprehensive income (loss) before reclassifications, net of tax			(2)		3
Amortization of net actuarial gain/loss and prior service cost/benefit					1
Total amount reclassified from Accumulated other comprehensive loss, net of tax					1
Total Other comprehensive income (loss)			(2)		4
Foreign currency translation
Balance at beginning of period	10		(1,053)		(983)
Other comprehensive income (loss)			(35)	10	(105)
Balance at end of period	10			10
Cash flow hedges (M)
Balance at beginning of period			(1)
Net amount reclassified to earnings			1
Total amount reclassified from Accumulated other comprehensive loss, net of tax			1
Total Other comprehensive (loss) income			1
Total Accumulated other comprehensive (loss) income	$ 10			$ 10
Non-controlling Interest [Member] | Interest Rate Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings			1
Pension and Other Postretirement Benefits attributable to Alcoa Corporation [Member]
Pension and other postretirement benefits (L)
Acquisition of noncontrolling interest (C)			(11)		(11)
Pension and Other Postretirement Benefits attributable to Non-controlling interest [Member]
Pension and other postretirement benefits (L)
Acquisition of noncontrolling interest (C)			11		11
Foreign Currency Translation attributable to Alcoa Corporation [Member]
Pension and other postretirement benefits (L)
Acquisition of noncontrolling interest (C)			(1,088)		(1,088)
Foreign Currency Translation attributable to Non-controlling Interest [Member]
Pension and other postretirement benefits (L)
Acquisition of noncontrolling interest (C)			$ 1,088		$ 1,088