Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and other postretirement benefits
Balance at beginning of period	$ (2,568)
Total Other comprehensive (loss) income	(220)	$ (482)	$ (2)
Balance at end of period	(2,644)	(2,568)
Foreign currency translation
Other comprehensive income (loss)	254	450	(1,995)
Available-for-sale securities
Other comprehensive income (loss)	(134)	137	(5)
Cash flow hedges
Total Other comprehensive (loss) income	26	(612)	826
Accumulated Other Comprehensive Loss
Pension and other postretirement benefits
Balance at beginning of period	(2,010)	(3,611)	(3,601)
Unrecognized net actuarial loss and prior service cost/benefit	(466)	(1,112)	(478)
Tax benefit (expense)	102	380	170
Total Other comprehensive (loss) income before reclassifications, net of tax	(364)	(732)	(308)
Amortization of net actuarial loss and prior service cost/benefit	222	389	458
Tax expense	(78)	(136)	(160)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	144	253	298
Total Other comprehensive (loss) income	(220)	(479)	(10)
Transfer to Alcoa Corporation	0	2,080	0
Balance at end of period	(2,230)	(2,010)	(3,611)
Foreign currency translation
Balance at beginning of period	(689)	(2,412)	(846)
Other comprehensive income (loss)	252	268	(1,566)
Transfer to Alcoa Corporation	0	1,455	0
Balance at end of period	(437)	(689)	(2,412)
Available-for-sale securities
Balance at beginning of period	132	(5)	0
Other comprehensive income (loss)	(134)	137	(5)
Transfer to Alcoa Corporation	0	0	0
Balance at end of period	(2)	132	(5)
Cash flow hedges
Balance at beginning of period	(1)	597	(230)
Net change from periodic revaluations	37	(843)	1,138
Tax benefit (expense)	(9)	252	(340)
Total Other comprehensive (loss) income before reclassifications, net of tax	28	(591)	798
Net amount reclassified to earnings	(3)	(38)	35
Tax benefit (expense)	1	12	(6)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(2)	(26)	29
Total Other comprehensive (loss) income	26	(617)	827
Transfer to Alcoa Corporation	0	19	0
Balance at end of period	25	(1)	597
Accumulated Other Comprehensive Loss | Aluminum Contracts
Cash flow hedges
Net amount reclassified to earnings	(2)	1	21
Accumulated Other Comprehensive Loss | Energy Contracts
Cash flow hedges
Net amount reclassified to earnings	0	(49)	6
Accumulated Other Comprehensive Loss | Foreign Exchange Contracts
Cash flow hedges
Net amount reclassified to earnings	0	0	5
Accumulated Other Comprehensive Loss | Interest Rate Contracts
Cash flow hedges
Net amount reclassified to earnings	0	9	1
Accumulated Other Comprehensive Loss | Nickel Contracts
Cash flow hedges
Net amount reclassified to earnings	(1)	1	2
Noncontrolling Interests
Pension and other postretirement benefits
Balance at beginning of period	0	(56)	(64)
Unrecognized net actuarial loss and prior service cost/benefit	0	(9)	5
Tax benefit (expense)	0	3	(1)
Total Other comprehensive (loss) income before reclassifications, net of tax	0	(6)	4
Amortization of net actuarial loss and prior service cost/benefit	0	4	6
Tax expense	0	(1)	(2)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	3	4
Total Other comprehensive (loss) income	0	(3)	8
Transfer to Alcoa Corporation	0	59	0
Balance at end of period	0	0	(56)
Foreign currency translation
Balance at beginning of period	(2)	(780)	(351)
Other comprehensive income (loss)	2	182	(429)
Transfer to Alcoa Corporation	0	596	0
Balance at end of period	0	(2)	(780)
Available-for-sale securities
Balance at beginning of period	0	0	0
Other comprehensive income (loss)	0	0	0
Transfer to Alcoa Corporation	0	0	0
Balance at end of period	0	0	0
Cash flow hedges
Balance at beginning of period	0	(3)	(2)
Net change from periodic revaluations	0	36	(1)
Tax benefit (expense)	0	(10)	0
Total Other comprehensive (loss) income before reclassifications, net of tax	0	26	(1)
Net amount reclassified to earnings	0	(29)	0
Tax benefit (expense)	0	8	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	(21)	0
Total Other comprehensive (loss) income	0	5	(1)
Transfer to Alcoa Corporation	0	(2)	0
Balance at end of period	0	0	(3)
Noncontrolling Interests | Aluminum Contracts
Cash flow hedges
Net amount reclassified to earnings	0	0	0
Noncontrolling Interests | Energy Contracts
Cash flow hedges
Net amount reclassified to earnings	0	(34)	0
Noncontrolling Interests | Foreign Exchange Contracts
Cash flow hedges
Net amount reclassified to earnings	0	0	0
Noncontrolling Interests | Interest Rate Contracts
Cash flow hedges
Net amount reclassified to earnings	0	5	0
Noncontrolling Interests | Nickel Contracts
Cash flow hedges
Net amount reclassified to earnings	$ 0	$ 0	$ 0