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BORROWINGS (Details) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Daily average outstanding $ 124,111,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding $ 13,059,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding
Total interest cost 108,000us-gaap_InterestExpenseBorrowings 48,000us-gaap_InterestExpenseBorrowings
Average interest rate paid (in hundredths) 0.35%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd 0.37%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd
Maximum amount outstanding at any month-end 155,000,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd 130,000,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd
Ending, balance 90,000,000us-gaap_AdvancesFromFederalHomeLoanBanks 130,000,000us-gaap_AdvancesFromFederalHomeLoanBanks
Weighted-average interest rate on balances outstanding (in hundredths) 0.34%us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresWeightedAverageInterestRate 0.32%us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresWeightedAverageInterestRate
Assets pledged as collateral to the Federal Home Loan Bank 567,000,000us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 505,000,000us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Federal Home Loan Bank borrowings 90,000,000subk_FederalHomeLoanBankBorrowings 130,000,000subk_FederalHomeLoanBankBorrowings
Federal Home Loan Bank Borrowings [Member]    
Debt Instrument [Line Items]    
Daily average outstanding 124,111,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember		 13,051,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
Total interest cost 108,000us-gaap_InterestExpenseBorrowings
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember		 48,000us-gaap_InterestExpenseBorrowings
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
Average interest rate paid (in hundredths) 0.35%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember		 0.37%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
Maximum amount outstanding at any month-end 155,000,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember		 130,000,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
Ending, balance 90,000,000us-gaap_AdvancesFromFederalHomeLoanBanks
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember		 130,000,000us-gaap_AdvancesFromFederalHomeLoanBanks
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
Weighted-average interest rate on balances outstanding (in hundredths) 0.34%us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember		 0.32%us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
Federal Funds Purchased [Member]    
Debt Instrument [Line Items]    
Daily average outstanding 0us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember		 8,000us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember
Total interest cost 0us-gaap_InterestExpenseBorrowings
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember		 0us-gaap_InterestExpenseBorrowings
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember
Average interest rate paid (in hundredths) 0.00%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember		 0.46%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember
Maximum amount outstanding at any month-end 0us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember		 0us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember
Ending, balance $ 0us-gaap_AdvancesFromFederalHomeLoanBanks
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember		 $ 0us-gaap_AdvancesFromFederalHomeLoanBanks
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember
Weighted-average interest rate on balances outstanding (in hundredths) 0.00%us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember		 0.00%us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalFundsPurchasedMember