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Real Estate Owned - Summary of Real Estate Owned Activity (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Real Estate Properties [Line Items]      
Balance, beginning of year $ 2,503us-gaap_RealEstateGrossAtCarryingValue    
Transfers into real estate owned 945us-gaap_RealEstateAcquisitionsThroughForeclosures    
Sale of real estate owned (2,095)us-gaap_RealEstateCostOfRealEstateSold    
Write down of real estate owned (831)us-gaap_SECScheduleIIIRealEstateWritedownOrReserveAmount (75)us-gaap_SECScheduleIIIRealEstateWritedownOrReserveAmount (1,433)us-gaap_SECScheduleIIIRealEstateWritedownOrReserveAmount
Balance, end of year 522us-gaap_RealEstateGrossAtCarryingValue 2,503us-gaap_RealEstateGrossAtCarryingValue  
Commercial Properties [Member]      
Real Estate Properties [Line Items]      
Balance, beginning of year 1,227us-gaap_RealEstateGrossAtCarryingValue
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_CommercialRealEstateMember		    
Sale of real estate owned (373)us-gaap_RealEstateCostOfRealEstateSold
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_CommercialRealEstateMember		    
Write down of real estate owned (764)us-gaap_SECScheduleIIIRealEstateWritedownOrReserveAmount
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_CommercialRealEstateMember		    
Balance, end of year 90us-gaap_RealEstateGrossAtCarryingValue
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_CommercialRealEstateMember		    
Residential Properties [Member]      
Real Estate Properties [Line Items]      
Balance, beginning of year 1,276us-gaap_RealEstateGrossAtCarryingValue
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ResidentialRealEstateMember		    
Transfers into real estate owned 567us-gaap_RealEstateAcquisitionsThroughForeclosures
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ResidentialRealEstateMember		    
Sale of real estate owned (1,722)us-gaap_RealEstateCostOfRealEstateSold
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ResidentialRealEstateMember		    
Write down of real estate owned (67)us-gaap_SECScheduleIIIRealEstateWritedownOrReserveAmount
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ResidentialRealEstateMember		    
Balance, end of year 54us-gaap_RealEstateGrossAtCarryingValue
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ResidentialRealEstateMember		    
Bank Properties [Member]      
Real Estate Properties [Line Items]      
Transfers into real estate owned 378us-gaap_RealEstateAcquisitionsThroughForeclosures
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= snbc_BankPropertiesMember		    
Sale of real estate owned 0us-gaap_RealEstateCostOfRealEstateSold
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= snbc_BankPropertiesMember		    
Balance, end of year $ 378us-gaap_RealEstateGrossAtCarryingValue
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= snbc_BankPropertiesMember