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COMMITMENTS, CREDIT RISK AND CONTINGENCIES - Additional Information (Detail)	Jun. 30, 2019
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