Income Taxes - Reconciliation of U.S. Federal Statutory Rate to Arconic's Effective Tax Rate (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. federal statutory rate		21.00%	21.00%	21.00%
Foreign tax rate differential		(1.40%)	10.60%	3.20%
U.S. and residual tax on foreign earnings		5.60%	15.30%	5.50%
U.S. State and local taxes		2.20%	0.80%	(0.40%)
Federal (cost) benefit of state tax		(2.00%)	1.20%	0.40%
Permanent differences related to asset disposals and items included in restructuring and other charges		6.80%	(1.30%)	(34.30%)
Non-deductible officer compensation		(3.50%)	(4.90%)	(0.30%)
Statutory tax rate and law changes		(15.90%)	(0.60%)	13.20%
Tax holidays		(0.40%)	(8.20%)	(3.00%)
Changes in valuation allowances		74.80%	(52.20%)	(1.30%)
Changes in uncertain tax positions		(116.90%)	0.30%	26.20%
Prior year tax adjustments		(1.70%)	44.30%	(4.20%)
Other		1.00%	3.90%	1.20%
Effective tax rate		(23.40%)	40.00%	27.80%
Foreign recapture				$ 74
Tax law change		$ 30
Income tax expense benefit	$ 272			59
Favorable spanish tax case decision		104
Foreign tax credits amount released			$ 112
Reserve liability		64
Tax position		$ 30
Foreign reserve				$ 38
Write off			112
Tax benefit deduction			$ 24

Deferred Tax Assets, Valuation Allowance, Favorable Spanish Tax Case Decision

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to income exempt from income tax because of a tax holiday.

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