LOANS AND LEASES - Troubled Debt Restructurings (Detail) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019 USD ($) Contract	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2019 USD ($) Contract	Jun. 30, 2018 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	41	229	81	447
Pre- Modification Outstanding Recorded Investment	$ 8	$ 4	$ 11	$ 8
Post- Modification Outstanding Recorded Investment	$ 8	$ 4	$ 10	$ 6
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	11	1	12	1
Pre- Modification Outstanding Recorded Investment	$ 3	$ 0	$ 4	$ 0
Post- Modification Outstanding Recorded Investment	$ 4	$ 0	$ 4	$ 0
Commercial and Industrial [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	2	13	13	13
Pre- Modification Outstanding Recorded Investment	$ 4	$ 1	$ 5	$ 3
Post- Modification Outstanding Recorded Investment	$ 3	$ 1	$ 4	$ 2
Total Commercial Loans and Leases [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	13	14	25	14
Pre- Modification Outstanding Recorded Investment	$ 7	$ 1	$ 9	$ 3
Post- Modification Outstanding Recorded Investment	$ 7	$ 1	$ 8	$ 2
Direct Installment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	14	178	32	357
Pre- Modification Outstanding Recorded Investment	$ 1	$ 2	$ 1	$ 3
Post- Modification Outstanding Recorded Investment	$ 1	$ 2	$ 1	$ 3
Residential Mortgages [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	7	8	10	19
Pre- Modification Outstanding Recorded Investment	$ 0	$ 0	$ 1	$ 1
Post- Modification Outstanding Recorded Investment	$ 0	$ 0	$ 1	$ 1
Indirect Installment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract		7		16
Pre- Modification Outstanding Recorded Investment		$ 0		$ 0
Post- Modification Outstanding Recorded Investment		$ 0		$ 0
Consumer Lines of Credit [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	7	22	14	41
Pre- Modification Outstanding Recorded Investment	$ 0	$ 1	$ 0	$ 1
Post- Modification Outstanding Recorded Investment	$ 0	$ 1	$ 0	$ 0
Consumer Loan [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts \| Contract	28	215	56	433
Pre- Modification Outstanding Recorded Investment	$ 1	$ 3	$ 2	$ 5
Post- Modification Outstanding Recorded Investment	$ 1	$ 3	$ 2	$ 4

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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnb_CommercialandIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnb_DirectInstallmentPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnb_IndirectInstallmentPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: