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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension and other postretirement benefits
Total Other comprehensive income	$ 185	$ 122	$ 287	$ 165
Foreign currency translation
Other comprehensive (loss)	(596)	(136)	(609)	217
Cash flow hedges
Total Other comprehensive income (loss) before reclassifications, net of tax	12		34
Total Other comprehensive income (loss)	(185)	(27)	345	(252)
Alcoa Corporation [Member]
Pension and other postretirement benefits
Balance at beginning of period	(2,685)	(2,286)	(2,786)	(2,330)
Unrecognized net actuarial loss and prior service cost/benefit	1	53	76	48
Tax (expense) benefit	6	2	(2)	3
Total Other comprehensive income before reclassifications, net of tax	7	55	74	51
Amortization of net actuarial loss and prior service cost/benefit	224	49	260	99
Tax expense	(48)	(2)	(50)	(4)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	176	47	210	95
Total Other comprehensive income	183	102	284	146
Balance at end of period	(2,502)	(2,184)	(2,502)	(2,184)
Foreign currency translation
Balance at beginning of period	(1,466)	(1,416)	(1,467)	(1,655)
Other comprehensive (loss)	(445)	(99)	(444)	140
Balance at end of period	(1,911)	(1,515)	(1,911)	(1,515)
Cash flow hedges
Balance at beginning of period	(379)	(98)	(929)	210
Net change from periodic revaluations	(231)	(50)	404	(430)
Tax (expense) benefit	31	19	(68)	77
Total Other comprehensive income (loss) before reclassifications, net of tax	(200)	(31)	336	(353)
Net amount reclassified to earnings	27	29	40	46
Tax (expense) benefit	(2)	(4)	(1)	(7)
Total amount reclassified from Accumulated other comprehensive (loss) income, net of tax	25	25	39	39
Total Other comprehensive income (loss)	(175)	(6)	375	(314)
Balance at end of period	(554)	(104)	(554)	(104)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	34	31	61	49
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	(7)	(2)	(20)	(3)
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges
Net amount reclassified to earnings			(1)
Non-controlling Interest [Member]
Pension and other postretirement benefits
Balance at beginning of period	(46)	(57)	(47)	(56)
Unrecognized net actuarial loss and prior service cost/benefit	2	19	3	18
Tax (expense) benefit			(1)
Total Other comprehensive income before reclassifications, net of tax	2	19	2	18
Amortization of net actuarial loss and prior service cost/benefit		1	1	1
Total amount reclassified from Accumulated other comprehensive loss, net of tax		1	1	1
Total Other comprehensive income	2	20	3	19
Balance at end of period	(44)	(37)	(44)	(37)
Foreign currency translation
Balance at beginning of period	(595)	(563)	(581)	(677)
Other comprehensive (loss)	(151)	(37)	(165)	77
Balance at end of period	(746)	(600)	(746)	(600)
Cash flow hedges
Balance at beginning of period	31	84	51	1
Net change from periodic revaluations	(10)	(30)	(30)	90
Tax (expense) benefit	3	9	9	(27)
Total Other comprehensive income (loss) before reclassifications, net of tax	(7)	(21)	(21)	63
Net amount reclassified to earnings	(4)	(1)	(13)	(2)
Tax (expense) benefit	1	1	4	1
Total amount reclassified from Accumulated other comprehensive (loss) income, net of tax	(3)		(9)	(1)
Total Other comprehensive income (loss)	(10)	(21)	(30)	62
Balance at end of period	21	63	21	63
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	$ (4)	$ (1)	$ (13)	$ (2)