Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,585	$ 4,924	$ 5,141
Adoption of accounting standard (A)				$ 73	$ 0	$ 0
Total Other comprehensive (loss) income, net of tax	(401)	85	(74)
Balance at end of period	4,621	5,585	4,924
Arconic
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,926)	(2,644)	(2,568)
Adoption of accounting standard (A)				(2)	(367)
Total Other comprehensive (loss) income, net of tax	(401)	85	(76)
Balance at end of period	(3,329)	(2,926)	(2,644)
Pension and other postretirement benefits (F)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,344)	(2,230)	(2,010)
Unrecognized net actuarial gain and prior service cost/benefit	(587)	70	(466)
Tax benefit (expense)	129	(19)	102
Total Other comprehensive (loss) income before reclassifications, net of tax	(458)	51	(364)
Amortization of net actuarial loss and prior service cost	90	262	222
Tax expense	(20)	(58)	(78)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	70	204	144
Total Other comprehensive (loss) income, net of tax	(388)	255	(220)
Balance at end of period	(2,732)	(2,344)	(2,230)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(583)	(437)	(689)
Total Other comprehensive (loss) income, net of tax	(13)	(146)	252
Balance at end of period	(596)	(583)	(437)
Debt securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(3)	(2)	132
Total Other comprehensive (loss) income, net of tax	3	(1)	(134)
Balance at end of period	0	(3)	(2)
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	4	25	(1)
Unrecognized net actuarial gain and prior service cost/benefit	(9)	(15)	37
Tax benefit (expense)	3	3	(9)
Total Other comprehensive (loss) income before reclassifications, net of tax	(6)	(12)	28
Amortization of net actuarial loss and prior service cost	4	(14)	(3)
Tax expense	(1)	3	1
Total amount reclassified from Accumulated other comprehensive loss, net of tax	3	(11)	(2)
Total Other comprehensive (loss) income, net of tax	(3)	(23)	26
Balance at end of period	(1)	4	25
Cash flow hedges | Aluminum Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amortization of net actuarial loss and prior service cost	5	(8)	(2)
Cash flow hedges | Interest Rate Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amortization of net actuarial loss and prior service cost	0	(2)	0
Cash flow hedges | Nickel Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amortization of net actuarial loss and prior service cost	(1)	(4)	(1)
Noncontrolling Interests
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0
Balance at end of period	0	0	0
Pension and other postretirement benefits (F)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	0
Unrecognized net actuarial gain and prior service cost/benefit	0	0	0
Tax benefit (expense)	0	0	0
Total Other comprehensive (loss) income before reclassifications, net of tax	0	0	0
Amortization of net actuarial loss and prior service cost	0	0	0
Tax expense	0	0	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	0	0
Total Other comprehensive (loss) income, net of tax	0	0	0
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	0	(2)
Total Other comprehensive (loss) income, net of tax	0	0	2
Balance at end of period	0	0	0
Debt securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	0
Total Other comprehensive (loss) income, 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	0
Unrecognized net actuarial gain and prior service cost/benefit	0	0	0
Tax benefit (expense)	0	0	0
Total Other comprehensive (loss) income before reclassifications, net of tax	0	0	0
Amortization of net actuarial loss and prior service cost	0	0	0
Tax expense	0	0	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	0	0
Total Other comprehensive (loss) income, net of tax	0	0	0
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]
Amortization of net actuarial loss and prior service cost	0	0	0
Cash flow hedges | Interest Rate Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amortization of net actuarial loss and prior service cost	0	0	0
Cash flow hedges | Nickel Contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amortization of net actuarial loss and prior service cost	$ 0	$ 0	$ 0
ASU 2017-07 | Pension and other postretirement benefits (F)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)				0	(369)	0
ASU 2017-07 | Pension and other postretirement benefits (F)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)				0	0	$ 0
ASU 2017-12
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)				2
ASU 2017-12 | Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)				(2)	2
ASU 2017-12 | Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of accounting standard (A)				$ 0	$ 0