Mergers And Acquisitions (Fair Value Of Net Assets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012
Mergers And Acquisitions [Abstract]
Cash and due from banks	$ 203,538
Securities	486,233
Loans	922,056
Goodwill and other intangible assets	116,527
Accrued income and other assets	117,611
Total assets	1,845,965
Deposits	1,525,253
Borrowings	171,606
Accrued expenses and other liabilities	12,288
Total liabilities	1,709,147
Purchase price	$ 136,818

Business acquisition purchase price allocation accrued income and other assets.

No definition available.

Name:	fnb_BusinessAcquisitionPurchasePriceAllocationAccruedIncomeAndOtherAssets
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business acquisition purchase price allocation borrowings.

+ References

No definition available.

+ Details

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

- Definition

Business Acquisition Purchase Price Allocation Deposits Acquired

+ References

No definition available.

+ Details

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

- Definition

Business acquisition purchase price allocation goodwill and other intangible assets.

+ References

No definition available.

+ Details

Name:	fnb_BusinessAcquisitionPurchasePriceAllocationGoodwillAndOtherIntangibleAssets
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business Acquisition Purchase Price Allocation Loans Receivable Acquired

+ References

No definition available.

+ Details

Name:	fnb_BusinessAcquisitionPurchasePriceAllocationLoansReceivableAcquired
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of acquisition cost of a business combination allocated to assets acquired.

+ References

+ Details

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

- Definition

The total purchase price of the acquired entity. This includes cash paid to equity interest holders of the acquired entity, fair value of debt and equity securities issued to equity holders of the acquired entity, and transaction costs paid to third parties to consummate the acquisition.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 35

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

The amount of cash and cash equivalents acquired in a business combination.

+ References

+ Details

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

- Definition

The amount of acquisition cost of a business combination allocated to marketable securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph e

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 37

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

The amount of acquisition cost of a business combination allocated to accrued expenses of the acquired entity.

+ References

+ Details

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

- Definition

The amount of acquisition cost of a business combination allocated to liabilities assumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 37

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 98-1

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Details

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