Business acquisition number of equity interests to be acquired.

No definition available.

Name:	fnb_BusinessAcquisitionNumberOfEquityInterestsToBeAcquired
Namespace Prefix:	fnb_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the amount of the core deposit intangible asset recorded as a result of the business combination.

+ References

No definition available.

+ Details

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

- Definition

Represents the fair value of deposits acquired in a business combination.

+ References

No definition available.

+ Details

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

- Definition

Exchange ratio used to determine the number of shares to be issued in conjunction with a business combination.

+ References

No definition available.

+ Details

Name:	fnb_BusinessAcquisitionShareConversionRatio
Namespace Prefix:	fnb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the number of shares of common stock exchanged by the acquired entity for common stock of the reporting entity.

+ References

No definition available.

+ Details

Name:	fnb_CommonStockAcquiredShares
Namespace Prefix:	fnb_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the number of bank branches acquired as a result of business combination.

+ References

No definition available.

+ Details

Name:	fnb_NumberOfBankBranchesAcquired
Namespace Prefix:	fnb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)(4)

-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

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Data Type:	xbrli:sharesItemType
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

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the fair value of the receivables acquired, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=56956791&loc=d3e4845-128472

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 30

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 30

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of assets acquired at the acquisition date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=56956791&loc=d3e4845-128472

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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- Details

Name:	us-gaap_BusinessAcquisitionAxis=fnb_ObaFinancialServicesMember
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Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fnb_MetroBancorpIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fnb_BankOfAmericaMember
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- Details

Name:	us-gaap_BusinessAcquisitionAxis=fnb_FifthThirdBankMember
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Mergers and Acquisitions - Additional Information (Detail) $ in Thousands	Sep. 18, 2015 USD ($) Branch	Sep. 03, 2015 USD ($) Branch	Aug. 04, 2015 USD ($) shares	Sep. 19, 2014 USD ($) shares	Feb. 15, 2014 USD ($) shares	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill recorded as a result of merger						$ 834,141	$ 832,213
BCSB Bancorp, Inc. (BCSB) [Member]
Business Acquisition [Line Items]
Deposits acquired as a result of merger					$ 532,197
Goodwill recorded as a result of merger					42,451
Core deposit intangibles recorded as result of the acquisition					6,591
Assets acquired as a result of merger					596,122
Loans acquired as a result of merger					304,932
Value of acquisition					$ 80,547
Common shares issued \| shares					6,730,597
Common shares acquired \| shares					3,235,961
OBA Financial Services, Inc. (OBA) [Member]
Business Acquisition [Line Items]
Deposits acquired as a result of merger				$ 295,922
Goodwill recorded as a result of merger				20,107
Core deposit intangibles recorded as result of the acquisition				4,304
Assets acquired as a result of merger				390,160
Loans acquired as a result of merger				291,393
Value of acquisition				$ 85,554
Common shares issued \| shares				7,170,037
Common shares acquired \| shares				4,025,895
Metro Bancorp Inc [Member]
Business Acquisition [Line Items]
Assets acquired as a result of merger			$ 3,001,357
Value of acquisition			$ 474,000
Common shares issued \| shares			33,600,000
Number of shares entitled to receive as per Merger agreement, Ratio			2.373
Common shares to be acquired \| shares			14,100,000
Bank of America [Member]
Business Acquisition [Line Items]
Deposits acquired as a result of merger	$ 154,619
Number of branch-banking locations acquired \| Branch	5
Goodwill recorded as a result of merger	$ 2,539
Core deposit intangibles recorded as result of the acquisition	$ 3,081
Fifth Third Bank [Member]
Business Acquisition [Line Items]
Deposits acquired as a result of merger		$ 383,000
Number of branch-banking locations acquired \| Branch		17