Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 4,924	$ 5,141	$ 14,131
Adoption of accounting standard (A)				$ 0
Total Other comprehensive income (loss), net of tax	85	(74)	(507)
Balance at end of period	5,585	4,924	5,141
Arconic
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,644)	(2,568)	(5,431)
Adoption of accounting standard (A)				(367)
Total Other comprehensive income (loss), net of tax	85	(76)	(691)
Balance at end of period	(2,926)	(2,644)	(2,568)
Pension and other postretirement benefits (G)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,230)	(2,010)	(3,611)
Other comprehensive income (loss), before reclassifications, before tax	70	(466)	(1,112)
Other comprehensive income (loss) before reclassifications, tax benefit (expense)	(19)	102	380
Total other comprehensive (loss) income before reclassifications, net of tax	51	(364)	(732)
Reclassification from accumulated other comprehensive income, before tax	262	222	389
Reclassification from AOCI, tax benefit (expense)	(58)	(78)	(136)
Total amount reclassified from accumulated other comprehensive loss, net of tax	204	144	253
Total Other comprehensive income (loss), net of tax	255	(220)	(479)
Transfer to Alcoa Corporation	0	0	2,080
Balance at end of period	(2,344)	(2,230)	(2,010)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(437)	(689)	(2,412)
Total Other comprehensive income (loss), net of tax	(146)	252	268
Transfer to Alcoa Corporation	0	0	1,455
Balance at end of period	(583)	(437)	(689)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2)	132	(5)
Total Other comprehensive income (loss), net of tax	(1)	(134)	137
Balance at end of period	(3)	(2)	132
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	25	(1)	597
Other comprehensive income (loss), before reclassifications, before tax	(15)	37	(843)
Other comprehensive income (loss) before reclassifications, tax benefit (expense)	3	(9)	252
Total other comprehensive (loss) income before reclassifications, net of tax	(12)	28	(591)
Reclassification from accumulated other comprehensive income, before tax	(14)	(3)	(38)
Reclassification from AOCI, tax benefit (expense)	3	1	12
Total amount reclassified from accumulated other comprehensive loss, net of tax	(11)	(2)	(26)
Total Other comprehensive income (loss), net of tax	(23)	26	(617)
Transfer to Alcoa Corporation	0	0	19
Balance at end of period	4	25	(1)
Cash flow hedges | Aluminum Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	(8)	(2)	1
Cash flow hedges | Energy Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	0	0	(49)
Cash flow hedges | Interest Rate Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	(2)	0	9
Cash flow hedges | Nickel Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	(4)	(1)	1
Noncontrolling Interests
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(2)
Balance at end of period	0	0	(2)
Pension and other postretirement benefits (G)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	(56)
Other comprehensive income (loss), before reclassifications, before tax	0	0	(9)
Other comprehensive income (loss) before reclassifications, tax benefit (expense)	0	0	3
Total other comprehensive (loss) income before reclassifications, net of tax	0	0	(6)
Reclassification from accumulated other comprehensive income, before tax	0	0	4
Reclassification from AOCI, tax benefit (expense)	0	0	(1)
Total amount reclassified from accumulated other comprehensive loss, net of tax	0	0	3
Total Other comprehensive income (loss), net of tax	0	0	(3)
Transfer to Alcoa Corporation	0	0	59
Balance at end of period	0	0	0
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(2)	(780)
Total Other comprehensive income (loss), net of tax	0	2	182
Transfer to Alcoa Corporation	0	0	596
Balance at end of period	0	0	(2)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	0
Total Other comprehensive income (loss), net of tax	0	0	0
Balance at end of period	0	0	0
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	(3)
Other comprehensive income (loss), before reclassifications, before tax	0	0	36
Other comprehensive income (loss) before reclassifications, tax benefit (expense)	0	0	(10)
Total other comprehensive (loss) income before reclassifications, net of tax	0	0	26
Reclassification from accumulated other comprehensive income, before tax	0	0	(29)
Reclassification from AOCI, tax benefit (expense)	0	0	8
Total amount reclassified from accumulated other comprehensive loss, net of tax	0	0	(21)
Total Other comprehensive income (loss), net of tax	0	0	5
Transfer to Alcoa Corporation	0	0	(2)
Balance at end of period	0	0	0
Cash flow hedges | Aluminum Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	0	0	0
Cash flow hedges | Energy Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	0	0	(34)
Cash flow hedges | Interest Rate Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	0	0	5
Cash flow hedges | Nickel Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification from accumulated other comprehensive income, before tax	$ 0	0	0
ASU 2017-07 | Pension and other postretirement benefits (G)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)		0	0	(369)
ASU 2017-07 | Pension and other postretirement benefits (G)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)		0	$ 0	0
ASU 2017-12 | Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)		$ 0		$ 2