Debt - Short-Term Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Short-term debt	$ 29	$ 38
Accounts payable settlement arrangements	$ 29	$ 33

Short-Term Debt, Excluding Current Maturities Of Long-Term Debt

No definition available.

Name:	arnc_ShortTermDebtExcludingCurrentMaturitiesOfLongtermDebt
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant