Revenue from Contracts with Customers - Narrative (Details) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Revenue from Contract with Customer [Abstract]
Revenue, shipping payment terms	30 days
Customer claims for new product introductions	$ 38

Contract with Customer, Claims for New Product Introductions

No definition available.

Name:	arnc_ContractwithCustomerClaimsforNewProductIntroductions
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration