Loans and Leases - Reserve for Commercial TDRs Included in Allowance for Credit Losses (Parenthetical) (Detail) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Receivables [Abstract]
Valuation for impairment of loans with pooled reserves	$ 500,000	$ 500,000

Threshold for valuation of commercial loans to be evaluated for pooled reserves.

No definition available.

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