Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Pension and other postretirement benefits (L)
Total Other comprehensive income (loss)	$ (256)	$ 125	$ (388)	$ 87
Foreign currency translation
Other comprehensive income	255	229	19	(679)
Cash flow hedges (M)
Net change from periodic revaluations	(397)	(513)	(700)	339
Net amount reclassified to earnings	67	13	119	38
Total Accumulated other comprehensive loss	(5,687)		(5,687)		$ (5,629)
Alcoa Corporation [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(2,405)	(2,244)	(2,536)	(2,282)
Unrecognized net actuarial loss and prior service cost/benefit	164	(181)	233	(201)
Tax benefit		4	2	10
Total Other comprehensive income (loss) before reclassifications, net of tax	164	(177)	235	(191)
Amortization of net actuarial loss and prior service cost/benefit	89	52	150	106
Tax expense		(2)	(1)	(4)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	89	50	149	102
Total Other comprehensive income (loss)	253	(127)	384	(89)
Balance at end of period	(2,152)	(2,371)	(2,152)	(2,371)
Foreign currency translation
Balance at beginning of period	(2,561)	(2,823)	(2,385)	(2,160)
Other comprehensive income	204	135	28	(528)
Balance at end of period	(2,357)	(2,688)	(2,357)	(2,688)
Cash flow hedges (M)
Balance at beginning of period	(912)	169	(708)	(532)
Net change from periodic revaluations	(397)	(513)	(700)	339
Tax benefit (expense)	75	112	131	(63)
Total Other comprehensive (loss) income before reclassifications, net of tax	(322)	(401)	(569)	276
Net amount reclassified to earnings	67	13	119	38
Tax (expense) benefit	(11)	(2)	(20)	(3)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	56	11	99	35
Total Other comprehensive (loss) income	(266)	(390)	(470)	311
Balance at end of period	(1,178)	(221)	(1,178)	(221)
Total Accumulated other comprehensive loss	(5,687)	(5,280)	(5,687)	(5,280)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	72	1	113	14
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	(4)	4	5	7
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	1	1	4	2
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	(2)	7	(3)	15
Non-controlling Interest [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(66)	(56)	(67)	(56)
Unrecognized net actuarial loss and prior service cost/benefit	2	1	2
Total Other comprehensive income (loss) before reclassifications, net of tax	2	1	2
Amortization of net actuarial loss and prior service cost/benefit	1	1	2	2
Total amount reclassified from Accumulated other comprehensive loss, net of tax	1	1	2	2
Total Other comprehensive income (loss)	3	2	4	2
Balance at end of period	(63)	(54)	(63)	(54)
Foreign currency translation
Balance at beginning of period	(904)	(1,079)	(844)	(834)
Other comprehensive income	51	94	(9)	(151)
Balance at end of period	(853)	(985)	(853)	(985)
Cash flow hedges (M)
Balance at beginning of period	(4)		(1)	20
Net change from periodic revaluations	9	(4)	(1)	(30)
Tax benefit (expense)	(3)	1		8
Total Other comprehensive (loss) income before reclassifications, net of tax	6	(3)	(1)	(22)
Net amount reclassified to earnings	(2)	3	4	1
Tax (expense) benefit	1	(1)	(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(1)	2	3	1
Total Other comprehensive (loss) income	5	(1)	2	(21)
Balance at end of period	1	(1)	1	(1)
Total Accumulated other comprehensive loss	(915)	(1,040)	(915)	(1,040)
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	$ (2)	$ 3	$ 4	$ 1