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INCOME TAXES (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
INCOME TAXES [Abstract]      
Income tax expense $ 1,241,000us-gaap_IncomeTaxExpenseBenefit $ 1,111,000us-gaap_IncomeTaxExpenseBenefit  
Effective tax rate (in hundredths) 23.60%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 23.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Unrecognized tax benefits 34,000us-gaap_UnrecognizedTaxBenefits 34,000us-gaap_UnrecognizedTaxBenefits 34,000us-gaap_UnrecognizedTaxBenefits
Unrecognized tax benefits accrued interest 0us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued    
Operating Loss Carryforwards [Line Items]      
Valuation reserve 1,000,000us-gaap_DeferredTaxAssetsValuationAllowance    
Operating loss carryforwards, expiration dates Dec. 31, 2032    
Income Tax Reconciliation, Change in Enacted Tax Rate   462,000us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate  
Income Tax Reconciliation, Nondeductible Expense, Share-based Compensation Cost   454,000us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost  
Federal [Member]      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards 0us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
New York State [Member]      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards 23,000,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Corporate tax rate, effective (in hundredths) 7.10%subk_EffectiveStateIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Corporate tax rate, future - 2016 (in hundredths) 6.50%subk_FutureApprovedEffectiveIncomeTaxRateReconciliationAtStateStatutoryIncomeTaxRate
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Asset qualifying threshold for tax modification 8,000,000,000subk_IncomeTaxModificationMaximumCorporateAssets
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Maximum mortgage threshold for loans $ 5,000,000subk_IncomeTaxModificationLoanMaximumThreshold
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember