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LOANS, Loans by Internal Assigned Grade for Credit Risk (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Credit risk profile by internally assigned grade [Abstract]    
Total $ 1,382,160subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome $ 1,355,427subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
Total (in hundredths) 100.00%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage 100.00%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage
Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 1,338,134subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
1,315,247subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Total (in hundredths) 96.80%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
97.00%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 18,938subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
10,407subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Total (in hundredths) 1.40%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0.80%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 25,088subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
29,773subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Total (in hundredths) 1.80%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
2.20%subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncomeInPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Commercial and Industrial [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 178,812subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
177,813subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Commercial and Industrial [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 165,527subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
167,922subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Commercial and Industrial [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 3,543subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
1,225subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Commercial and Industrial [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 9,742subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
8,666subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Commercial Real Estate [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 579,873subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
560,524subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Commercial Real Estate [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 553,861subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
536,536subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Commercial Real Estate [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 15,395subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
9,182subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Commercial Real Estate [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 10,617subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
14,806subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Multifamily [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 322,229subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
309,666subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
Multifamily [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 322,229subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
309,666subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Multifamily [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Multifamily [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MultifamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Mixed Use Commercial [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 35,333subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
34,806subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
Mixed Use Commercial [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 35,313subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
34,806subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Mixed Use Commercial [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Mixed Use Commercial [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 20subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_MixedUseCommercialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Real Estate Construction [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 24,608subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
26,206subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
Real Estate Construction [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 24,608subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
26,206subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Real Estate Construction [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Real Estate Construction [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= subk_RealEstateConstructionPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Residential Mortgages [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 184,977subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
187,828subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
Residential Mortgages [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 181,809subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
183,263subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Residential Mortgages [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Residential Mortgages [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 3,168subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
4,565subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Home Equity [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 49,440subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
50,982subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
Home Equity [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 48,021subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
49,569subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Home Equity [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Home Equity [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 1,419subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
1,413subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Consumer [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 6,888subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
7,602subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Consumer [Member] | Pass [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 6,766subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
7,279subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Consumer [Member] | Special Mention [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total 0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Consumer [Member] | Substandard [Member]    
Credit risk profile by internally assigned grade [Abstract]    
Total $ 122subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
$ 323subk_LoansAndLeasesReceivableExcludingOtherLoansNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember