LOANS AND LEASES - Schedule of Loan Modifications (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 21.8	$ 5.4	$ 63.0	$ 16.4
Term Extension
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	13.6	2.6	34.8	10.0
Term Extension and Rate Reduction
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	1.0	0.7	13.6	2.0
Other
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	7.2	2.1	14.6	4.4
Commercial real estate \| Term Extension
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 1.1	$ 1.0	$ 18.1	$ 2.7
% of Total Class of Financing Receivable	0.01%	0.01%	0.15%	0.02%
Term increase from modification	9 months	10 months	13 months	5 months
Commercial real estate \| Term Extension and Rate Reduction
Financing Receivable, Modified [Line Items]
Amortized Cost Basis			$ 11.6
% of Total Class of Financing Receivable			0.10%
Term increase from modification			15 months
Interest rate reduction from modification			3.00%
Commercial real estate \| Other
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 3.2	$ 0.8	$ 6.2	$ 2.2
% of Total Class of Financing Receivable	0.03%	0.01%	0.05%	0.02%
Term of modified payment deferral				3 months
Commercial real estate \| Other \| Minimum
Financing Receivable, Modified [Line Items]
Term of modified payment deferral	3 months		3 months
Commercial real estate \| Other \| Maximum
Financing Receivable, Modified [Line Items]
Term of modified payment deferral	5 months		5 months
Commercial and industrial \| Term Extension
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 11.5	$ 0.3	$ 11.8	$ 1.2
% of Total Class of Financing Receivable	0.13%	0.00%	0.13%	0.02%
Term increase from modification	3 months	72 months	4 months	41 months
Commercial and industrial \| Term Extension and Rate Reduction
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 0.1		$ 0.1	$ 0.1
% of Total Class of Financing Receivable	0.00%		0.00%	0.00%
Commercial and industrial \| Other
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 4.0	$ 1.3	$ 8.1	$ 2.2
% of Total Class of Financing Receivable	0.05%	0.02%	0.09%	0.03%
Term of modified payment deferral				3 months
Commercial and industrial \| Other \| Minimum
Financing Receivable, Modified [Line Items]
Term of modified payment deferral	3 months		3 months
Commercial and industrial \| Other \| Maximum
Financing Receivable, Modified [Line Items]
Term of modified payment deferral	6 months		6 months
Direct installment \| Term Extension
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 0.5	$ 0.4	$ 1.2	$ 1.9
% of Total Class of Financing Receivable	0.02%	0.01%	0.05%	0.07%
Term increase from modification	14 months	36 months	16 months	26 months
Residential mortgages \| Term Extension
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 0.3	$ 0.7	$ 3.5	$ 3.6
% of Total Class of Financing Receivable	0.00%	0.01%	0.04%	0.04%
Term increase from modification	78 months	117 months	46 months	44 months
Residential mortgages \| Term Extension and Rate Reduction
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 0.9	$ 0.7	$ 1.9	$ 1.9
% of Total Class of Financing Receivable	0.01%	0.01%	0.02%	0.02%
Term increase from modification			132 months	28 years
Interest rate reduction from modification			2.13%
Residential mortgages \| Term Extension and Rate Reduction \| Minimum
Financing Receivable, Modified [Line Items]
Interest rate reduction from modification				1.00%
Residential mortgages \| Term Extension and Rate Reduction \| Maximum
Financing Receivable, Modified [Line Items]
Interest rate reduction from modification				4.75%
Consumer lines of credit \| Term Extension
Financing Receivable, Modified [Line Items]
Amortized Cost Basis	$ 0.2	$ 0.2	$ 0.2	$ 0.6
% of Total Class of Financing Receivable	0.01%	0.01%	0.01%	0.04%
Term increase from modification	243 months	129 months	225 months	165 months
Consumer lines of credit \| Other
Financing Receivable, Modified [Line Items]
Amortized Cost Basis			$ 0.3
% of Total Class of Financing Receivable			0.02%

Financing Receivable, Modified, Payment Deferral, Term

No definition available.

Name:	fnb_FinancingReceivableModifiedPaymentDeferralTerm
Namespace Prefix:	fnb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- 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/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage of decrease in weighted-average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of increase in weighted-average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityAndInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=fnb_OtherLoanRestructuringModificationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
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:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnb_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnb_DirectInstallmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnb_ResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnb_ConsumerLinesOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: