VARIABLE INTEREST ENTITIES - Affordable Housing Tax Credit Investments And Related Unfunded Commitments (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
LIHTC investments included in other assets	$ 81	$ 86
Unfunded LIHTC commitments	$ 62	$ 37

Qualified Affordable Housing Project Investments, Unfunded Commitments

No definition available.

Name:	fnb_QualifiedAffordableHousingProjectInvestmentsUnfundedCommitments
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant