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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Pension and other postretirement benefits (L)
Total Other comprehensive (loss) income	$ (125)	$ 8	$ (87)	$ 50
Foreign currency translation
Other comprehensive income	229	44	(679)	24
Cash flow hedges (M)
Net change from periodic revaluations	(513)	80	339	(272)
Net amount reclassified to earnings	13	10	38	1
Total Accumulated other comprehensive loss	(5,280)		(5,280)		$ (4,974)
Alcoa Corporation [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(2,244)	(2,242)	(2,282)	(2,283)
Unrecognized net actuarial loss and prior service cost/benefit	(181)	(78)	(201)	(82)
Tax benefit (expense)	4	16	10	17
Total Other comprehensive (loss) income before reclassifications, net of tax	(177)	(62)	(191)	(65)
Amortization of net actuarial loss and prior service cost/benefit	52	83	106	128
Tax expense	(2)	(11)	(4)	(12)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	50	72	102	116
Total Other comprehensive (loss) income	(127)	10	(89)	51
Balance at end of period	(2,371)	(2,232)	(2,371)	(2,232)
Foreign currency translation
Balance at beginning of period	(2,823)	(2,093)	(2,160)	(2,071)
Other comprehensive income	135	40	(528)	18
Balance at end of period	(2,688)	(2,053)	(2,688)	(2,053)
Cash flow hedges (M)
Balance at beginning of period	169	(499)	(532)	(211)
Net change from periodic revaluations	(513)	80	339	(272)
Tax benefit (expense)	112	(12)	(63)	54
Total Other comprehensive (loss) income before reclassifications, net of tax	(401)	68	276	(218)
Net amount reclassified to earnings	13	10	38	1
Tax (expense) benefit	(2)	1	(3)	8
Total amount reclassified from Accumulated other comprehensive loss, net of tax	11	11	35	9
Total Other comprehensive (loss) income	(390)	79	311	(209)
Balance at end of period	(221)	(420)	(221)	(420)
Total Accumulated other comprehensive loss	(5,280)	(4,705)	(5,280)	(4,705)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	1	12	14	25
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	4	(6)	7	(32)
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	1		2
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	7	4	15	8
Non-controlling Interest [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(56)	(45)	(56)	(46)
Unrecognized net actuarial loss and prior service cost/benefit	1	(3)		(3)
Total Other comprehensive (loss) income before reclassifications, net of tax	1	(3)		(3)
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 (loss) income	2	(2)	2	(1)
Balance at end of period	(54)	(47)	(54)	(47)
Foreign currency translation
Balance at beginning of period	(1,079)	(808)	(834)	(810)
Other comprehensive income	94	4	(151)	6
Balance at end of period	(985)	(804)	(985)	(804)
Cash flow hedges (M)
Balance at beginning of period		37	20	31
Net change from periodic revaluations	(4)	6	(30)	33
Tax benefit (expense)	1	(2)	8	(10)
Total Other comprehensive (loss) income before reclassifications, net of tax	(3)	4	(22)	23
Net amount reclassified to earnings	3	(7)	1	(25)
Tax (expense) benefit	(1)	2		7
Total amount reclassified from Accumulated other comprehensive loss, net of tax	2	(5)	1	(18)
Total Other comprehensive (loss) income	(1)	(1)	(21)	5
Balance at end of period	(1)	36	(1)	36
Total Accumulated other comprehensive loss	(1,040)	(815)	(1,040)	(815)
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	$ 3	$ (7)	$ 1	$ (25)