Borrowings - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Credit available with FHLB	$ 4,213,966
Credit with FHLB used	900,018fnb_FederalHomeLoanBankCreditUsed
Federal Home Loan Bank advances are scheduled to mature periodically through the year	2021
Effective interest rates, minimum	0.76%	0.76%
Effective interest rates, maximum	4.19%	4.19%
Number of unconsolidated subsidiary trusts	2
Percent of the common equity of each Trust owned by the Corporation	100.00%
TPS Outstanding amount	58,259	58,246
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Debt Instrument [Line Items]
Percentage of TPS moving from tier 1 capital to tier 2 capital for 2015	25.00%
TPS Outstanding amount	$ 57,500
Percentage of TPS moving from tier 1 capital to tier 2 capital for 2016	75.00%

Amount of advances or credit lines used from the Federal Home Loan Bank (FHLB).

No definition available.

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

- Definition

Number of unconsolidated subsidiary trusts owned by the company.

+ References

No definition available.

+ Details

Name:	fnb_NumberOfUnconsolidatedSubsidiaryTrusts
Namespace Prefix:	fnb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of common equity of each unconsolidated subsidiary trust owned by the company.

+ References

No definition available.

+ Details

Name:	fnb_PercentageOfEachTrustCommonEquityOwned
Namespace Prefix:	fnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage Of Trust Preferred Securities Moving From Tier One To Tier Two Capital In Next Rolling Twelve Months

+ References

No definition available.

+ Details

Name:	fnb_PercentageOfTrustPreferredSecuritiesMovingFromTierOneToTierTwoCapitalInNextRollingTwelveMonths
Namespace Prefix:	fnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage Of Trust Preferred Securities Moving From Tier One To Tier Two Capital In Rolling Year Two

+ References

No definition available.

+ Details

Name:	fnb_PercentageOfTrustPreferredSecuritiesMovingFromTierOneToTierTwoCapitalInRollingYearTwo
Namespace Prefix:	fnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Definition

The last year of maturity for the advances from the Federal Home Loan Bank as of the Balance Sheet date.

+ References

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresDueDateLast
Namespace Prefix:	us-gaap_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	instant

- Definition

States the lowest interest rate applicable to any of the advances from the Federal Home Loan Bank as of the Balance Sheet date.

+ References

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresInterestRateRangeFrom
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

States the highest interest rate applicable to any of the advances from the Federal Home Loan Bank as of the Balance Sheet date.

+ References

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresInterestRateRangeTo
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of maximum advances or credit lines available from the Federal Home Loan Bank (FHLB).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 470

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Subparagraph b

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13(3)(a))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22 (b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13

-Subparagraph 3(a)

-Article 9

+ Details

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

- Definition

Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Details

Name:	us-gaap_ScheduleOfSharesSubjectToMandatoryRedemptionBySettlementTermsAxis=us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: