VARIABLE INTEREST ENTITIES - Affordable Housing Tax Credit Investments And Related Unfunded Commitments (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
LIHTC investments included in other assets	$ 87	$ 87
Unfunded LIHTC commitments	$ 31	$ 34

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