Income Taxes - Reconciliation of U.S. Federal Statutory Rate to Arconic's Effective Tax Rate (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. federal statutory rate			21.00%	21.00%	35.00%
Foreign tax rate differential			2.60%	2.40%	(8.70%)
U.S. and residual tax on foreign earnings			6.00%	1.60%	(0.10%)
U.S. State and local taxes			2.50%	1.50%	0.70%
Federal benefit of state tax			0.40%	(0.30%)	3.70%
Permanent differences on restructuring and other charges and asset disposals			(22.90%)	(16.90%)	(167.40%)
Non-deductible transaction costs			1.60%	0.00%	0.30%
Non-deductible officer compensation			(1.80%)	(0.10%)	0.00%
Statutory tax rate and law changes			(0.20%)	6.50%	52.50%
Tax holidays			(3.20%)	(1.60%)	(3.00%)
Changes in valuation allowances			(14.20%)	0.90%	137.90%
Impairment of goodwill			0.00%	0.00%	53.50%
Changes in uncertain tax positions			6.10%	12.80%	10.10%
Prior year tax adjustments			15.20%	(2.60%)	(0.90%)
Other			1.60%	0.60%	2.10%
Effective tax rate			18.30%	26.00%	115.70%
Tax cuts and jobs act of 2017, incomplete accounting, provisional income tax expense (benefit)	$ 272	$ 272		$ 59