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Derivative Instruments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Derivative [Line Items]    
Derivative assets $ 45,898us-gaap_DerivativeAssets $ 33,317us-gaap_DerivativeAssets
Derivative liabilities 46,160us-gaap_DerivativeLiabilities 43,369us-gaap_DerivativeLiabilities
Fair value of derivatives, in a net liability position 45,090us-gaap_DerivativeNetLiabilityPositionAggregateFairValue 38,239us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Collateral with derivative counterparties, fair value 33,247us-gaap_CollateralAlreadyPostedAggregateFairValue 37,427us-gaap_CollateralAlreadyPostedAggregateFairValue
Cash collateral posted 16,061fnb_CashCollateralAlreadyPostedAggregateFairValue 1,976fnb_CashCollateralAlreadyPostedAggregateFairValue
Additional amount in excess of posted collateral required in case of breached agreements 1,862us-gaap_AdditionalCollateralAggregateFairValue 2,224us-gaap_AdditionalCollateralAggregateFairValue
Commercial Borrower Derivatives [Member]    
Derivative [Line Items]    
Notional amount 973,212invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= fnb_CommercialBorrowerDerivativesMember		 828,576invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= fnb_CommercialBorrowerDerivativesMember
Risk Management Derivatives [Member]    
Derivative [Line Items]    
Notional amount 200,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= fnb_RiskManagementDerivativesMember		 200,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= fnb_RiskManagementDerivativesMember
Derivative, Gain (Loss) on Derivative, Net 0us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= fnb_RiskManagementDerivativesMember		 0us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= fnb_RiskManagementDerivativesMember
Trading Assets [Member]    
Derivative [Line Items]    
Derivative assets 43,789us-gaap_DerivativeAssets
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_TradingAccountAssetsMember		 33,317us-gaap_DerivativeAssets
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_TradingAccountAssetsMember
Not for Trading Assets [Member]    
Derivative [Line Items]    
Derivative assets 2,109us-gaap_DerivativeAssets
/ us-gaap_FairValueByAssetClassAxis
= fnb_NonTradingAccountAssetsMember		 0us-gaap_DerivativeAssets
/ us-gaap_FairValueByAssetClassAxis
= fnb_NonTradingAccountAssetsMember
Trading Liabilities [Member]    
Derivative [Line Items]    
Derivative liabilities 43,830us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_TradingLiabilitiesMember		 33,236us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_TradingLiabilitiesMember
Not for Trading Liabilities [Member]    
Derivative [Line Items]    
Derivative liabilities $ 2,330us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByLiabilityClassAxis
= fnb_NonTradingLiabilitiesMember		 $ 10,133us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByLiabilityClassAxis
= fnb_NonTradingLiabilitiesMember