RETIREMENT PLANS - Additional Information (Detail) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) Retirement_Plan	Jun. 30, 2018 USD ($)	Sep. 30, 2018 Retirement_Plan
Retirement Benefits [Abstract]
Percent of employer match			100.00%
Employee contribution percentage			6.00%
Employee contribution		$ 0.9
Additional discretionary contribution, annual financial goals, percentage			3.00%
Number of supplemental non qualified retirement plans \| Retirement_Plan	2		2
Employer contribution, defined benefit plan	$ 4.0

Percentage of discretionary contributions made by an employer to a defined contribution plan.

No definition available.

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Data Type:	num:percentItemType
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Period Type:	duration

Defined Contribution Plan, Employer Discretionary Non-contribution Amount

No definition available.

Name:	fnb_DefinedContributionPlanEmployerDiscretionaryNoncontributionAmount
Namespace Prefix:	fnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Number of supplemental non-qualified retirement plans sponsored by the entity.

No definition available.

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

No definition available.

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Data Type:	xbrli:stringItemType
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Period Type:	duration

Amount of discretionary contributions made by an employer to a defined contribution plan.

No definition available.

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

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Percentage employer matches of the employee's percentage contribution matched.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration