COMMITMENTS, CREDIT RISK AND CONTINGENCIES - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Percentage of commitments to extend credit dependent upon the financial condition of the customers	70.70%
Allowance for credit losses on loan commitments that are not unconditionally cancellable	$ 18.0	$ 13.7

Percentage of commitments to extend credit that are dependent of the financial condition of the customer.

No definition available.

Name:	fnb_PercentageOfCommitmentsToExtendCreditDependentOnFinancialCondition
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant