Securities - Schedule of Amortized Cost and Fair Value of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	$ 1,534,771	$ 1,169,482	$ 1,170,439
Securities Available For Sale, Gross Unrealized Gains	11,215	11,463	16,858
Securities Available For Sale, Gross Unrealized Losses	(11,921)	(39,295)	(14,614)
Securities Available For Sale, Fair Value	1,532,718	1,139,524
Securities Available For Sale, Fair Value	1,347	2,126
Securities Available For Sale, Fair Value	1,534,065us-gaap_AvailableForSaleSecurities	1,141,650us-gaap_AvailableForSaleSecurities	1,172,683us-gaap_AvailableForSaleSecurities
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	29,604
Securities Available For Sale, Gross Unrealized Gains	78
Securities Available For Sale, Fair Value	29,682
U.S. Government-Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	338,330	336,763	352,910
Securities Available For Sale, Gross Unrealized Gains	742	126	1,676
Securities Available For Sale, Gross Unrealized Losses	(1,939)	(5,904)	(129)
Securities Available For Sale, Fair Value	337,133	330,985	354,457
Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	546,572	247,880	267,575
Securities Available For Sale, Gross Unrealized Gains	7,548	4,304	7,575
Securities Available For Sale, Gross Unrealized Losses	(35)	(1,303)
Securities Available For Sale, Fair Value	554,085	250,881	275,150
Securities Available For Sale, Fair Value	554,085
Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	580,601	511,098	465,574
Securities Available For Sale, Gross Unrealized Gains	1,617	895	4,201
Securities Available For Sale, Gross Unrealized Losses	(9,047)	(20,794)	(228)
Securities Available For Sale, Fair Value	573,171	491,199	469,547
Securities Available For Sale, Fair Value	573,171
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	1,414	1,747	2,679
Securities Available For Sale, Gross Unrealized Gains	17	15	50
Securities Available For Sale, Fair Value	1,431	1,762	2,729
Securities Available For Sale, Fair Value	1,431
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	7,891
Securities Available For Sale, Gross Unrealized Losses	(11)
Securities Available For Sale, Fair Value	7,880
Securities Available For Sale, Fair Value	7,880
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	12,713	16,842	23,592
Securities Available For Sale, Gross Unrealized Gains	477	410	1,232
Securities Available For Sale, Gross Unrealized Losses	(32)	(250)
Securities Available For Sale, Fair Value	13,158	17,002	24,824
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost		37,203	34,765
Securities Available For Sale, Gross Unrealized Gains		4,507	967
Securities Available For Sale, Gross Unrealized Losses		(10,115)	(13,276)
Securities Available For Sale, Fair Value		31,595	22,456
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	16,615	16,505	21,790
Securities Available For Sale, Gross Unrealized Gains	420	524	695
Securities Available For Sale, Gross Unrealized Losses	(857)	(929)	(972)
Securities Available For Sale, Fair Value	16,178	16,100	21,513
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	1,533,740	1,168,038	1,168,885
Securities Available For Sale, Gross Unrealized Gains	10,899	10,781	16,396
Securities Available For Sale, Gross Unrealized Losses	(11,921)	(39,295)	(14,605)
Securities Available For Sale, Fair Value	1,532,718	1,139,524	1,170,676
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	1,031	1,444	1,554
Securities Available For Sale, Gross Unrealized Gains	316	682	462
Securities Available For Sale, Gross Unrealized Losses			(9)
Securities Available For Sale, Fair Value	1,347	2,126	2,007
Securities Available For Sale, Fair Value	$ 1,347

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

+ References

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

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

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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

+ Details

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

- Definition

Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

-Paragraph 2

-Subparagraph (b)-(g)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

+ Details

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

- Definition

Amount of equity securities categorized neither as held-to-maturity nor as trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

+ Details

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

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fnb_AgencyCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fnb_NonAgencyCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: