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Disclosures About Fair Value of Financial Instruments (Details 5) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Debt securities  
Quantitative information for Level 3 Fair Value Measurements Assets  
Fair value 10,306us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Discounted cash flow  
Quantitative information for Level 3 Fair Value Measurements Assets  
Default rates (as a percent) 1.00%nwbi_FairValueInputsDefaultRate
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DebtSecuritiesMember
/ us-gaap_ValuationTechniqueAxis
= nwbi_DiscountedCashFlowValuationTechniqueMember
Prepayment speeds (as a percent) 1.00%nwbi_FairValueInputsPrepaymentSpeeds
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DebtSecuritiesMember
/ us-gaap_ValuationTechniqueAxis
= nwbi_DiscountedCashFlowValuationTechniqueMember
Debt securities | Discounted cash flow | Minimum  
Quantitative information for Level 3 Fair Value Measurements Assets  
Discount margin (as a percent) 0.35%nwbi_FairValueInputsDiscountMargin
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DebtSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= nwbi_DiscountedCashFlowValuationTechniqueMember
Debt securities | Discounted cash flow | Maximum  
Quantitative information for Level 3 Fair Value Measurements Assets  
Discount margin (as a percent) 2.10%nwbi_FairValueInputsDiscountMargin
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DebtSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= nwbi_DiscountedCashFlowValuationTechniqueMember
Debt securities | Discounted cash flow | Weighted Average  
Quantitative information for Level 3 Fair Value Measurements Assets  
Discount margin (as a percent) 0.69%nwbi_FairValueInputsDiscountMargin
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DebtSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= nwbi_DiscountedCashFlowValuationTechniqueMember
Loans evaluated for impairment  
Quantitative information for Level 3 Fair Value Measurements Assets  
Fair value 52,926us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= nwbi_ImpairedLoansMember
Estimated costs to sell (as a percent) 10.00%nwbi_FairValueInputsEstimatedCostsToSell
/ us-gaap_FairValueByAssetClassAxis
= nwbi_ImpairedLoansMember
Real estate owned  
Quantitative information for Level 3 Fair Value Measurements Assets  
Fair value 15,346us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= nwbi_OtherRealEstateOwnedMember
Estimated costs to sell (as a percent) 10.00%nwbi_FairValueInputsEstimatedCostsToSell
/ us-gaap_FairValueByAssetClassAxis
= nwbi_OtherRealEstateOwnedMember