COMMITMENTS, CREDIT RISK AND CONTINGENCIES - Additional Information (Detail)	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Percentage of Commitments to extend credit dependent upon the financial condition of the customers	76.90%

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:	num:percentItemType
Balance Type:	na
Period Type:	instant