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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. federal statutory rate			21.00%	35.00%	35.00%
Taxes on foreign operations			4.00%	(8.80%)	(11.50%)
U.S. State and local taxes			1.50%	0.70%	(0.40%)
Federal benefit of state tax			(0.003)	0.037	0.004
Permanent differences on restructuring and other charges and asset disposals			(16.90%)	(167.40%)	(107.80%)
Non-deductible acquisition costs			0.00%	0.30%	8.40%
Statutory tax rate and law changes			6.50%	52.50%	(15.70%)
Tax holidays			(1.60%)	(3.00%)	(0.80%)
Changes in valuation allowances			0.90%	137.90%	426.80%
Impairment of goodwill			0.00%	53.50%	0.00%
Company-owned life insurance/split-dollar net premiums			(0.00%)	(0.00%)	23.00%
Changes in uncertain tax positions			12.80%	10.10%	2.10%
Prior year tax adjustments			(2.60%)	(0.90%)	(1.70%)
Other			0.70%	2.10%	(1.30%)
Effective tax rate			26.00%	115.70%	356.50%
Tax cuts and jobs act of 2017, incomplete accounting, provisional income tax expense (benefit)	$ 272	$ 272	$ 59