Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Pension and other postretirement benefits
Total Other comprehensive income (loss)	$ (13)	$ (402)	$ (63)	$ (689)
Foreign currency translation
Other comprehensive loss	(299)	(196)	(275)	(805)
Cash flow hedges
Total Accumulated other comprehensive loss	(4,849)		(4,849)		$ (4,565)
Alcoa Corporation [Member]
Pension and other postretirement benefits
Balance at beginning of period	(2,232)	(2,502)	(2,283)	(2,786)
Unrecognized net actuarial (loss) gain and prior service cost/benefit	(38)	174	(120)	250
Tax benefit (expense)	11	(1)	28	(3)
Total Other comprehensive (loss) income before reclassifications, net of tax	(27)	173	(92)	247
Amortization of net actuarial loss and prior service cost/benefit	49	227	177	487
Tax expense	(3)	(2)	(15)	(52)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	46	225	162	435
Total Other comprehensive income (loss)	19	398	70	682
Balance at end of period	(2,213)	(2,104)	(2,213)	(2,104)
Foreign currency translation
Balance at beginning of period	(2,053)	(1,911)	(2,071)	(1,467)
Other comprehensive loss	(224)	(142)	(206)	(586)
Balance at end of period	(2,277)	(2,053)	(2,277)	(2,053)
Cash flow hedges
Balance at beginning of period	(420)	(554)	(211)	(929)
Net change from periodic revaluations	60	(60)	(212)	344
Tax (expense) benefit	(15)	10	39	(58)
Total Other comprehensive income (loss) before reclassifications, net of tax	45	(50)	(173)	286
Net amount reclassified to earnings	13	23	14	63
Tax benefit (expense)	3	(2)	11	(3)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	16	21	25	60
Total Other comprehensive income (loss)	61	(29)	(148)	346
Balance at end of period	(359)	(583)	(359)	(583)
Total Accumulated other comprehensive loss	(4,849)	(4,740)	(4,849)	(4,740)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	9	26	34	87
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	(4)	(6)	(36)	(26)
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	4		4
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges
Net amount reclassified to earnings	4	3	12	2
Non-controlling Interest [Member]
Pension and other postretirement benefits
Balance at beginning of period	(47)	(44)	(46)	(47)
Unrecognized net actuarial (loss) gain and prior service cost/benefit	(11)	4	(14)	7
Tax benefit (expense)	4	(1)	4	(2)
Total Other comprehensive (loss) income before reclassifications, net of tax	(7)	3	(10)	5
Amortization of net actuarial loss and prior service cost/benefit	2	1	4	2
Tax expense	(1)		(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	1	1	3	2
Total Other comprehensive income (loss)	(6)	4	(7)	7
Balance at end of period	(53)	(40)	(53)	(40)
Foreign currency translation
Balance at beginning of period	(804)	(746)	(810)	(581)
Other comprehensive loss	(75)	(54)	(69)	(219)
Balance at end of period	(879)	(800)	(879)	(800)
Cash flow hedges
Balance at beginning of period	36	21	31	51
Net change from periodic revaluations	2	12	35	(18)
Tax (expense) benefit	(1)	(4)	(11)	5
Total Other comprehensive income (loss) before reclassifications, net of tax	1	8	24	(13)
Net amount reclassified to earnings	(6)	(4)	(31)	(17)
Tax benefit (expense)	2	1	9	5
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(4)	(3)	(22)	(12)
Total Other comprehensive income (loss)	(3)	5	2	(25)
Balance at end of period	33	26	33	26
Total Accumulated other comprehensive loss	(899)	(814)	(899)	(814)
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	$ (6)	$ (4)	$ (31)	$ (17)