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COMMITMENTS, CREDIT RISK AND CONTINGENCIES - Additional Information (Detail) | Jun. 30, 2019
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Commitments and Contingencies Disclosure [Abstract] |
Percentage of Commitments to extend credit dependent upon the financial condition of the customers | 73.50%

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- Definition Percentage of commitments to extend credit that are dependent of the financial condition of the customer. + References No definition available. + Details
Name: | fnb_PercentageOfCommitmentsToExtendCreditDependentOnFinancialCondition
Namespace Prefix: | fnb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration