ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Dec. 31, 2024
Financing Receivable, Allowance for Credit Loss [Line Items]
Percentage of increase, housing price index	4.00%		4.30%
Percentage of increase, commercial real estate price index	1.30%		(0.50%)
Percentage of increase (decline), S&P volatility in next fiscal year	17.50%		(2.20%)
Percentage of increase (decline), S&P volatility in next second year	(4.60%)		(7.90%)
ACL on loans and leases	$ 443.0		$ 439.0
Provision for credit losses	18.5	$ 17.5
Allowance for credit losses on loans and leases
Financing Receivable, Allowance for Credit Loss [Line Items]
ACL on loans and leases	443.0	428.9	$ 439.5	$ 422.8
Financing receivable, allowance for credit loss, period increase (decrease)	$ 3.5
Financing receivable, allowance for credit loss, period increase, percentage	0.80%
Financing receivable, allowance for credit loss, ratio	1.26%		1.26%
Provision for credit losses	$ 19.4	18.6
Charge-offs	$ 15.9	$ 12.5
Financing receivable, allowance for credit loss, net charge-offs (recovery), percentage	0.18%	0.15%

Financing Receivable, Allowance for Credit Loss, Ratio

No definition available.

Name:	fnb_FinancingReceivableAllowanceforCreditLossRatio
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Period Increase (Decrease), Percentage

+ References

No definition available.

+ Details

Name:	fnb_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossPeriodIncreaseDecreasePercentage
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff (Recovery), Percentage

+ References

No definition available.

+ Details

Name:	fnb_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoffRecoveryPercentage
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Forecasted Macroeconomic Variables, Percentage of Growth (Decline), Commercial Real Estate Price Index

+ References

No definition available.

+ Details

Name:	fnb_ForecastedMacroeconomicVariablesPercentageOfGrowthDeclineCommercialRealEstatePriceIndex
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Forecasted Macroeconomic Variables, Percentage of Growth (Decline), Housing Price Index

+ References

No definition available.

+ Details

Name:	fnb_ForecastedMacroeconomicVariablesPercentageOfGrowthDeclineHousingPriceIndex
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Forecasted Macroeconomic Variables, Percentage of Growth (Decline), S&P Volatility In Next Fiscal Year

+ References

No definition available.

+ Details

Name:	fnb_ForecastedMacroeconomicVariablesPercentageOfGrowthDeclineSPVolatilityInNextFiscalYear
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Forecasted Macroeconomic Variables, Percentage of Growth (Decline), S&P Volatility In Next Second Year

+ References

No definition available.

+ Details

Name:	fnb_ForecastedMacroeconomicVariablesPercentageOfGrowthDeclineSPVolatilityInNextSecondYear
Namespace Prefix:	fnb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, excluding accrued interest, of increase (decrease) in allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease (increase) in allowance for credit loss on financing receivable from writeoff (recovery). Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476148/220-40-50-22

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnb_AllowanceForCreditLossesOnLoansAndLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: