Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Pension and other postretirement benefits
Total Other comprehensive income	$ 42	$ 102
Foreign currency translation
Other comprehensive (loss) income	(20)	(13)
Cash flow hedges
Total Other comprehensive (loss) income	(282)	530
Total Accumulated other comprehensive loss	(4,834)		$ (4,565)
Alcoa Corporation [Member]
Pension and other postretirement benefits
Balance at beginning of period	(2,283)	(2,786)	(2,786)
Unrecognized net actuarial (loss) gain and prior service cost/benefit	(4)	75
Tax benefit (expense)	1	(8)
Total Other comprehensive (loss) income before reclassifications, net of tax	(3)	67
Amortization of net actuarial loss and prior service cost/benefit	45	36
Tax expense	(1)	(2)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	44	34
Total Other comprehensive income	41	101
Balance at end of period	(2,242)	(2,685)	(2,283)
Foreign currency translation
Balance at beginning of period	(2,071)	(1,467)	(1,467)
Other comprehensive (loss) income	(22)	1
Balance at end of period	(2,093)	(1,466)	(2,071)
Cash flow hedges
Balance at beginning of period	(211)	(929)	(929)
Net change from periodic revaluations	(352)	635
Tax benefit (expense)	66	(99)
Total Other comprehensive (loss) income before reclassifications, net of tax	(286)	536
Net amount reclassified to earnings	(9)	13
Tax benefit	7	1
Total amount reclassified from Accumulated other comprehensive (loss)income, net of tax	(2)	14
Total Other comprehensive (loss) income	(288)	550
Balance at end of period	(499)	(379)	(211)
Total Accumulated other comprehensive loss	(4,834)	(4,530)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	13	27
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	(26)	(13)
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges
Net amount reclassified to earnings	4	(1)
Non-controlling Interest [Member]
Pension and other postretirement benefits
Balance at beginning of period	(46)	(47)	(47)
Unrecognized net actuarial (loss) gain and prior service cost/benefit		1
Tax benefit (expense)		(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	1
Balance at end of period	(45)	(46)	(46)
Foreign currency translation
Balance at beginning of period	(810)	(581)	(581)
Other comprehensive (loss) income	2	(14)
Balance at end of period	(808)	(595)	(810)
Cash flow hedges
Balance at beginning of period	31	51	51
Net change from periodic revaluations	27	(20)
Tax benefit (expense)	(8)	6
Total Other comprehensive (loss) income before reclassifications, net of tax	19	(14)
Net amount reclassified to earnings	(18)	(9)
Tax benefit	5	3
Total amount reclassified from Accumulated other comprehensive (loss)income, net of tax	(13)	(6)
Total Other comprehensive (loss) income	6	(20)
Balance at end of period	37	31	$ 31
Total Accumulated other comprehensive loss	(816)	(610)
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	$ (18)	$ (9)