Fair Value Measurements (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Fair Value Measurements
Percentage of securities using market-based information	98.40%
Percentage of securities using model-based techniques	1.60%
Amount of total losses included in earnings relating to assets still held	$ 0	$ (2,281)	$ (2,281)
Impaired loans, carrying amount	10,137
Allocated allowance for loan losses	3,921
Fair value of allocated allowance	6,734
Estimated costs to sell	518
Provision fair value measurements included in allowance for loan losses	3,587
Carrying amount of OREO	17,362
Write down of OREO	12,850
Fair value of OREO	14,702
Estimated cost to sell OREO	1,852
Loss from OREO included in earnings	$ 4,512

Fair Value Disclosures [Abstract]

Name:	fnb_FairValueDisclosuresAbstract
Namespace Prefix:	fnb
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Gain Loss Included In Earnings Relating To Assets Still Held

+ Details

Name:	fnb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarningsRelatingToAssetsStillHeld
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value of OREO.

+ Details

Name:	fnb_FairValueOfOtherRealEstateOwned
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Loss from OREO included in earnings.

+ Details

Name:	fnb_GainsLossesIncludedInEarningOnNonRecurringAssetsWriteDown
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Impaired Loan Cost To Sell

+ Details

Name:	fnb_ImpairedLoanCostToSell
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Impaired loans.

+ Details

Name:	fnb_ImpairedLoans
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Other Real Estate Fair Value Disclsoure

+ Details

Name:	fnb_OtherRealEstateFairValueDisclsoure
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Estimated cost to sell OREO.

+ Details

Name:	fnb_OtherRealEstateOwnedCostToSell
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Write down of OREO.

+ Details

Name:	fnb_OtherRealEstateOwnedWrittenDownValue
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of securities using market based information.

+ Details

Name:	fnb_PercentageOfSecuritiesUsingMarketBasedInformation
Namespace Prefix:	fnb
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of securities using model based techniques.

+ Details

Name:	fnb_PercentageOfSecuritiesUsingModelBasedTechniques
Namespace Prefix:	fnb
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Provision For Loan And Lease Losses Fair Value Disclosure

+ Details

Name:	fnb_ProvisionForLoanAndLeaseLossesFairValueDisclosure
Namespace Prefix:	fnb
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reflects amount of allowance for credit losses pertaining to impaired loans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 114
-Paragraph 20
-Subparagraph a

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher OTS
-Name Federal Regulation (FR)
-Number Title 12
-Chapter V
-Section 563c.102
-Paragraph 8
-Subparagraph j(ii)
-Subsection I

+ Details

Name:	us-gaap_LoansAndLeasesReceivableImpairedAllowanceForLoanLosses
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the adjusted carrying amount of loans for which it is probable, based on current facts and circumstances, that a creditor will not initially be able to collect all amounts due according to the contractual terms of the loan agreement, or will not recover the previously reported carrying amount of the loan.

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableImpairedAtCarryingValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant