Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension and other postretirement benefits
Total Other comprehensive income (loss)	$ 504	$ (447)	$ (202)
Foreign currency translation
Other comprehensive (loss) income	(833)	284	315
Cash flow hedges
Total Other comprehensive income (loss) before reclassifications, net of tax	62	25
Total Other comprehensive income (loss)	698	(1,089)	(342)
Financial Contracts [Member]
Cash flow hedges
Total Other comprehensive income (loss) before reclassifications, net of tax			96
Alcoa Corporation [Member]
Pension and other postretirement benefits
Balance at beginning of period	(2,786)	(2,330)	(352)
Establishment of additional defined benefit plans			(2,704)
Separation-related adjustments			928
Unrecognized net actuarial loss and prior service cost/benefit	19	(671)	(307)
Tax (expense) benefit	(8)	25	6
Total Other comprehensive income (loss) before reclassifications, net of tax	11	(646)	(301)
Amortization of net actuarial loss and prior service cost/benefit	546	199	107
Tax expense	(54)	(9)	(8)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	492	190	99
Total Other comprehensive income (loss)	503	(456)	(202)
Balance at end of period	(2,283)	(2,786)	(2,330)
Foreign currency translation
Balance at beginning of period	(1,467)	(1,655)	(1,851)
Separation-related adjustments			(17)
Other comprehensive (loss) income	(604)	188	213
Balance at end of period	(2,071)	(1,467)	(1,655)
Cash flow hedges
Balance at beginning of period	(929)	210	603
Separation-related adjustments			(47)
Net change from periodic revaluations	803	(1,489)	(558)
Tax (expense) benefit	(159)	251	233
Total Other comprehensive income (loss) before reclassifications, net of tax	644	(1,238)	(325)
Net amount reclassified to earnings	77	109	(40)
Tax (expense) benefit	(3)	(10)	19
Total amount reclassified from Accumulated other comprehensive loss, net of tax	74	99	(21)
Total Other comprehensive income (loss)	718	(1,139)	(346)
Balance at end of period	(211)	(929)	210
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	108	130	7
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	(37)	(19)	(54)
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges
Net amount reclassified to earnings	6	(2)
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings			7
Non-controlling Interest [Member]
Pension and other postretirement benefits
Balance at beginning of period	(47)	(56)	(56)
Unrecognized net actuarial loss and prior service cost/benefit	(3)	9	2
Tax (expense) benefit		(2)	(6)
Total Other comprehensive income (loss) before reclassifications, net of tax	(3)	7	(4)
Amortization of net actuarial loss and prior service cost/benefit	4	2	5
Tax expense			(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	4	2	4
Total Other comprehensive income (loss)	1	9
Balance at end of period	(46)	(47)	(56)
Foreign currency translation
Balance at beginning of period	(581)	(677)	(779)
Other comprehensive (loss) income	(229)	96	102
Balance at end of period	(810)	(581)	(677)
Cash flow hedges
Balance at beginning of period	51	1	(3)
Net change from periodic revaluations	(4)	83	38
Tax (expense) benefit	1	(25)	(12)
Total Other comprehensive income (loss) before reclassifications, net of tax	(3)	58	26
Net amount reclassified to earnings	(24)	(12)	(32)
Tax (expense) benefit	7	4	10
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(17)	(8)	(22)
Total Other comprehensive income (loss)	(20)	50	4
Balance at end of period	31	51	1
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	$ (24)	$ (12)	(37)
Non-controlling Interest [Member] | Interest Rate Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings			$ 5