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Loans receivable (Details 9) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
item
Mar. 31, 2014
item
Troubled debt restructurings by type of modification    
Number of re-modified TDRs 1nwbi_FinancingReceivableModificationsNumberOfContractsRemodified 4nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
Total $ 84nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment $ 159nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
Rate    
Troubled debt restructurings by type of modification    
Total 84nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_InterestRateModificationMember		  
Maturity date    
Troubled debt restructurings by type of modification    
Total   136nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_MaturityDateModificationMember
Other    
Troubled debt restructurings by type of modification    
Total   23nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_OtherModificationMember
Personal Banking    
Troubled debt restructurings by type of modification    
Number of re-modified TDRs 1nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember		 1nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
Total 84nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember		 77nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
Personal Banking | Rate    
Troubled debt restructurings by type of modification    
Total 84nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_InterestRateModificationMember		  
Personal Banking | Maturity date    
Troubled debt restructurings by type of modification    
Total   77nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_MaturityDateModificationMember
Personal Banking | Residential mortgage loans    
Troubled debt restructurings by type of modification    
Number of re-modified TDRs   1nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
Total   77nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
Personal Banking | Residential mortgage loans | Maturity date    
Troubled debt restructurings by type of modification    
Total   77nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_MaturityDateModificationMember
Personal Banking | Home equity loans    
Troubled debt restructurings by type of modification    
Number of re-modified TDRs 1nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_HomeEquityMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember		  
Total 84nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_HomeEquityMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember		  
Personal Banking | Home equity loans | Rate    
Troubled debt restructurings by type of modification    
Total 84nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_HomeEquityMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_PersonalBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_InterestRateModificationMember		  
Business Banking    
Troubled debt restructurings by type of modification    
Number of re-modified TDRs   3nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
Total   82nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
Business Banking | Maturity date    
Troubled debt restructurings by type of modification    
Total   59nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_MaturityDateModificationMember
Business Banking | Other    
Troubled debt restructurings by type of modification    
Total   23nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_OtherModificationMember
Business Banking | Commercial real estate loans    
Troubled debt restructurings by type of modification    
Number of re-modified TDRs   2nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
Total   77nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
Business Banking | Commercial real estate loans | Maturity date    
Troubled debt restructurings by type of modification    
Total   59nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_MaturityDateModificationMember
Business Banking | Commercial real estate loans | Other    
Troubled debt restructurings by type of modification    
Total   18nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_OtherModificationMember
Business Banking | Commercial loans    
Troubled debt restructurings by type of modification    
Number of re-modified TDRs   1nwbi_FinancingReceivableModificationsNumberOfContractsRemodified
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
Total   5nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
Business Banking | Commercial loans | Other    
Troubled debt restructurings by type of modification    
Total   $ 5nwbi_FinancingReceivableRemodificationsPostModificationRecordedInvestment
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nwbi_BusinessBankingMember
/ nwbi_FinancingReceivableTroubledDebtRestructuringByTypesOfConcessionsMadeAxis
= nwbi_OtherModificationMember