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BUSINESS SEGMENTS - Additional Information (Detail) - Segment | 1 Months Ended | 6 Months Ended
--- | --- | ---
Aug. 31, 2018 | Jun. 30, 2019
Segment Reporting Information [Line Items] |  |
Number of reportable segments |  | 3
Mariner Finance, LLC [Member] |  |
Segment Reporting Information [Line Items] |  |
Stock purchase agreement, stock sold, percentage | 100.00% |

X
- Definition Stock Purchase Agreement, Stock Sold, Percentage + References No definition available. + Details
Name: | fnb_StockPurchaseAgreementStockSoldPercentage
Namespace Prefix: | fnb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_CounterpartyNameAxis=fnb_MarinerFinanceLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |