Loan Servicing - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Servicing Assets at Fair Value [Line Items]
Service charges	$ 24,807,000	$ 21,134,000
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Unpaid principal balance of loans	2,700,000,000		$ 1,800,000,000
Mortgage servicing fees recorded in mortgage banking income	1,600,000	900,000
Valuation allowance	0	$ 0
SBA-Guaranteed Loan Servicing [Member]
Servicing Assets at Fair Value [Line Items]
Unpaid principal balance of loans	255,200,000
Valuation allowance	0
Service charges	$ 300,000

Amount of unpaid principal balance related to loans serviced for others.

No definition available.

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

- Definition

Amount of fees and commissions from financial services and banking activities and correspondent clearing. Includes fees from depositor accounts, credit cards, merchant discounts, and fiduciary and trust activities, guarantee fees, investment advisory, management, and administrative fees, transfer agent fees, and insurance commissions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_FeesAndCommissions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income from mortgage banking activities (fees and commissions) and servicing real estate mortgages, credit cards, and other financial assets held by others net of any related impairment of capitalized service costs. Also includes any premiums received in lieu of regular servicing fees on such loans only as earned over the life of the loans.

+ References

No definition available.

+ Details

Name:	us-gaap_FeesAndCommissionsMortgageBankingAndServicing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in the amortized amount from a decrease (increase) in the valuation allowance applied to the servicing asset for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (a)(5)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122739-111746

+ Details

Name:	us-gaap_ServicingAssetAtAmortizedValueValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_ServicingAssetsAtFairValueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=fnb_MortgageLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fnb_SmallBusinessAdministrationLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: