Investments - Summary of Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Sales	$ 4,469	$ 3,507	$ 4,119
Cost of goods sold	3,781	2,844	3,209
(Loss) Income before income taxes	132	85	125
Net (loss) income	82	21	43
Equity in net (loss) income of affiliated companies, before reconciling adjustments	52	25	60
Other	10	3	(7)
Alcoa Corporation's equity in net (loss) income of affiliated companies	62	28	53
Current assets	1,862	1,396
Noncurrent assets	11,015	11,454
Current liabilities	1,699	1,767
Noncurrent liabilities	6,751	6,777
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Sales	3,032	1,970	2,025
Cost of goods sold	2,776	1,905	2,070
(Loss) Income before income taxes	(142)	(295)	(362)
Net (loss) income	(157)	(295)	(366)
Equity in net (loss) income of affiliated companies, before reconciling adjustments	(39)	(75)	(91)
Other	9	7	(2)
Alcoa Corporation's equity in net (loss) income of affiliated companies	(30)	(68)	(93)
Current assets	1,415	1,141
Noncurrent assets	9,373	9,653
Current liabilities	1,331	1,452
Noncurrent liabilities	6,191	6,204
Halco Mining [Member]
Schedule of Equity Method Investments [Line Items]
Sales	416	437	487
Cost of goods sold	266	242	236
(Loss) Income before income taxes	50	53	86
Net (loss) income	47	50	80
Equity in net (loss) income of affiliated companies, before reconciling adjustments	21	23	36
Other	(1)	2	(2)
Alcoa Corporation's equity in net (loss) income of affiliated companies	20	25	34
Current assets	40	12
Noncurrent assets	174	189
Current liabilities	1	4
Noncurrent liabilities	12	14
Energetica Barra Grande SA [Member]
Schedule of Equity Method Investments [Line Items]
Sales	131	60	130
Cost of goods sold	117	35	98
(Loss) Income before income taxes	13	16	27
Net (loss) income	5	15	7
Equity in net (loss) income of affiliated companies, before reconciling adjustments	2	6	3
Other		(1)	(1)
Alcoa Corporation's equity in net (loss) income of affiliated companies	2	5	2
Current assets	44	17
Noncurrent assets	285	302
Current liabilities	48	31
Noncurrent liabilities	6	23
Pechiney Reynolds Quebec Inc [Member]
Schedule of Equity Method Investments [Line Items]
Sales	332	309	486
Cost of goods sold	292	272	288
(Loss) Income before income taxes	35	36	113
Net (loss) income	23	16	104
Equity in net (loss) income of affiliated companies, before reconciling adjustments	11	8	52
Other		(4)	4
Alcoa Corporation's equity in net (loss) income of affiliated companies	11	4	56
Current assets	140	91
Noncurrent assets	106	147
Current liabilities	65	62
Noncurrent liabilities	12
Consorcio Serra Do Facao [Member]
Schedule of Equity Method Investments [Line Items]
Sales	103	90	84
Cost of goods sold	48	65	76
(Loss) Income before income taxes	45	8	6
Net (loss) income	46	5	(1)
Equity in net (loss) income of affiliated companies, before reconciling adjustments	16	2
Other			(2)
Alcoa Corporation's equity in net (loss) income of affiliated companies	16	2	(2)
Current assets	79	19
Noncurrent assets	261	451
Current liabilities	25	37
Noncurrent liabilities	117	283
MRN [Member]
Schedule of Equity Method Investments [Line Items]
Sales	350	451	498
Cost of goods sold	273	297	308
(Loss) Income before income taxes	35	152	118
Net (loss) income	30	129	98
Equity in net (loss) income of affiliated companies, before reconciling adjustments	6	23	18
Other		(1)	(3)
Alcoa Corporation's equity in net (loss) income of affiliated companies	6	22	15
Current assets	121	92
Noncurrent assets	744	644
Current liabilities	220	174
Noncurrent liabilities	413	253
Manicouagan Power Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Sales	105	104	106
Cost of goods sold	9	10	16
(Loss) Income before income taxes	96	94	91
Net (loss) income	88	87	90
Equity in net (loss) income of affiliated companies, before reconciling adjustments	35	35	36
Other	2
Alcoa Corporation's equity in net (loss) income of affiliated companies	37	35	36
Current assets	23	24
Noncurrent assets	72	68
Current liabilities	$ 9	7
Dampier to Bunbury Natural Gas Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Sales		86	303
Cost of goods sold		18	117
(Loss) Income before income taxes		21	46
Net (loss) income		14	31
Equity in net (loss) income of affiliated companies, before reconciling adjustments		3	6
Other			(1)
Alcoa Corporation's equity in net (loss) income of affiliated companies		$ 3	$ 5