SECURITIES - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Schedule Of Securities [Line Items]
Gross gains	$ 0	$ 0
Gross losses	0	0
Other than temporary impairment losses, investments, debt available-for-sale securities	0	$ 0
Municipal bond portfolio, value	$ 1,100,000,000
Percentage of formal credit enhancement insurance of municipalities	64.00%
Municipal Bonds [Member] \| Weighted Average [Member]
Schedule Of Securities [Line Items]
Average holding size of securities in bond portfolio	$ 3,200,000
Municipal Bonds [Member] \| Credit Concentration Risk [Member] \| General Obligation Bonds [Member] \| A Rating [Member] \| Minimum [Member]
Schedule Of Securities [Line Items]
Percentage of portfolio	100.00%
Municipal Bonds [Member] \| Credit Concentration Risk [Member] \| General Obligation Bonds [Member] \| A Rating or Better [Member]
Schedule Of Securities [Line Items]
Percentage of portfolio	99.00%
Municipal Bonds [Member] \| Geographic Concentration Risk [Member] \| Pennsylvania, Ohio and Maryland [Member]
Schedule Of Securities [Line Items]
Percentage of portfolio	66.00%

The average holding size, aggregate value, of the securities in the municipal bond portfolio.

No definition available.

Name:	fnb_AverageSecuritiesPortfolioSize
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Marketable Securities, Gross Realized Gains

+ References

No definition available.

+ Details

Name:	fnb_MarketableSecuritiesGrossRealizedGains
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Marketable Securities, Gross Realized Losses

+ References

No definition available.

+ Details

Name:	fnb_MarketableSecuritiesGrossRealizedLosses
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage as of the balance sheet date of municipal bonds that have formal credit enhancement insurance.

+ References

No definition available.

+ Details

Name:	fnb_MunicipalBondsPercentWithFormalCreditEnhancementInsurance
Namespace Prefix:	fnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Schedule Of Securities [Line Items]

+ References

No definition available.

+ Details

Name:	fnb_ScheduleOfSecuritiesLineItems
Namespace Prefix:	fnb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Bonds or similar securities issued by state, city, or local governments or the agencies operated by state, city, or local governments which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

No definition available.

+ Details

Name:	us-gaap_MunicipalDebtSecuritiesAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of other-than-temporary impairment (OTTI) on investment in debt security, recognized in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

+ Details

Name:	us-gaap_OtherThanTemporaryImpairmentLossDebtSecuritiesPortionRecognizedInEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CreditConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=fnb_GeneralObligationBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=fnb_ARatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=fnb_ARatingorBetterMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_GeographicConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=fnb_PaAndOhAndMdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: