Basis of Presentation - Additional Information (Detail) - AWAC [Member]	Sep. 30, 2019
Alumina Limited [Member]
Basis Of Presentation [Line Items]
Non-controlling interest, ownership percentage	40.00%
Alcoa Corporation [Member]
Basis Of Presentation [Line Items]
Ownership interest percentage	60.00%

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

No definition available.

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant