Allowance for Credit Losses - Additional Information (Detail) $ in Thousands	Jun. 30, 2015 USD ($)
Receivables [Abstract]
Minimum reserves for commercial loan	$ 500

Minimum amount for which the company evaluates impairment for individual loans.

No definition available.

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