Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Pension and other postretirement benefits (L)
Total Other comprehensive income	$ (132)	$ (38)
Foreign currency translation
Other comprehensive loss	(236)	(908)
Cash flow hedges (M)
Net change from periodic revaluations	(303)	852
Net amount reclassified to earnings	52	25
Total Accumulated other comprehensive loss	(5,878)		$ (5,629)
Alcoa Corporation [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(2,536)	(2,282)
Unrecognized net actuarial loss and prior service cost/benefit	69	(20)
Tax benefit	2	6
Total Other comprehensive income (loss) before reclassifications, net of tax	71	(14)
Amortization of net actuarial loss and prior service cost/benefit	61	54
Tax expense	(1)	(2)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	60	52
Total Other comprehensive income	131	38
Balance at end of period	(2,405)	(2,244)
Foreign currency translation
Balance at beginning of period	(2,385)	(2,160)
Other comprehensive loss	(176)	(663)
Balance at end of period	(2,561)	(2,823)
Cash flow hedges (M)
Balance at beginning of period	(708)	(532)
Net change from periodic revaluations	(303)	852
Tax benefit (expense)	56	(175)
Total Other comprehensive (loss) income before reclassifications, net of tax	(247)	677
Net amount reclassified to earnings	52	25
Tax (expense) benefit	(9)	(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	43	24
Total Other comprehensive (loss) income	(204)	701
Balance at end of period	(912)	169
Total Accumulated other comprehensive loss	(5,878)	(4,898)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	41	13
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	9	3
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	3	1
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	(1)	8
Non-controlling Interest [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(67)	(56)
Unrecognized net actuarial loss and prior service cost/benefit		(1)
Total Other comprehensive income (loss) before reclassifications, net of tax		(1)
Amortization of net actuarial loss and prior service cost/benefit	1	1
Total amount reclassified from Accumulated other comprehensive loss, net of tax	1	1
Total Other comprehensive income	1
Balance at end of period	(66)	(56)
Foreign currency translation
Balance at beginning of period	(844)	(834)
Other comprehensive loss	(60)	(245)
Balance at end of period	(904)	(1,079)
Cash flow hedges (M)
Balance at beginning of period	(1)	20
Net change from periodic revaluations	(10)	(26)
Tax benefit (expense)	3	7
Total Other comprehensive (loss) income before reclassifications, net of tax	(7)	(19)
Net amount reclassified to earnings	6	(2)
Tax (expense) benefit	(2)	1
Total amount reclassified from Accumulated other comprehensive loss, net of tax	4	(1)
Total Other comprehensive (loss) income	(3)	(20)
Balance at end of period	(4)
Total Accumulated other comprehensive loss	(974)	(1,135)
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	$ 6	$ (2)