Financing Receivable, Allowance for Credit Loss, Ratio

No definition available.

Name:	fnb_FinancingReceivableAllowanceforCreditLossRatio
Namespace Prefix:	fnb_
Data Type:	dtr-types1: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-types1: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

Financing Receivable, Excluding Accrued Interest, Credit Loss Expense (Reversal), Specific Reserves

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Financial Instrument Performance Status Change, Downgrade Amount

+ References

No definition available.

+ Details

Name:	fnb_FinancingReceivableFinancialInstrumentPerformanceStatusChangeDowngradeAmount
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Financing Receivable, Non-Purchased Credit Deterioration Loans, Allowance for Credit Loss at Acquisition Date, Expense (Reversal)

+ References

No definition available.

+ Details

Name:	fnb_FinancingReceivableNonPurchasedCreditDeteriorationLoansAllowanceForCreditLossAtAcquisitionDateExpenseReversal
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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-types1: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-types1: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-types1: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-types1: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/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

Reference 2: 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

+ 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

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
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_BusinessAcquisitionAxis=fnb_HowardBancorpIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 22, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Percentage of decline, housing price index		5.40%		5.40%			3.70%
Percentage of decline, commercial real estate price index		8.90%		8.90%			0.90%
Percentage of decline, S&P volatility in next fiscal year		33.20%		33.20%			41.00%
Percentage of decline, S&P volatility in next second year		21.00%		21.00%			8.10%
Allowance for credit loss		$ 413.0		$ 413.0			$ 402.0
Provision for credit losses		18.5	$ 6.4	32.6	$ 24.4
Financing receivable, provision expense, specific reserves		13.0
Non-Performing \| Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, financial instrument performance status change, downgrade amount		32.0		32.0
Howard
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, non-purchased credit deterioration loans, allowance for credit loss at acquisition date, provision	$ 19.1
Allowance for credit losses on loans and leases
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss		$ 412.7	378.0	412.7	378.0	$ 403.4	$ 401.7	$ 370.6	$ 344.3
Financing receivable, allowance for credit loss, period increase				$ 11.0
Financing receivable, allowance for credit loss, period increase, percentage				2.70%
Financing receivable, allowance for credit loss, ratio		1.32%		1.32%			1.33%
Provision for credit losses		$ 18.0	$ 7.0	$ 32.9	$ 25.2
Financing receivable, allowance for credit loss, net charge-offs (recovery), percentage		(0.11%)	0.01%	(0.14%)	(0.01%)
Financing receivable, allowance for credit loss, net charge-offs (recovery)		$ 8.7	$ (0.4)	$ 21.9	$ 1.5
Allowance for credit losses on loans and leases \| Howard
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, non-purchased credit deterioration loans, allowance for credit loss at acquisition date, provision	$ 19.1