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Borrowed Funds (Maturity and weighted average interest rates of FHLB advances) (Details) - Federal Home Loan Bank, Advances, Fixed Rate Due 2024 - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
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Federal Home Loan Bank, Advances [Line Items] |  |
FHLB fixed rate advances | $ 0 | $ 40,000
FHLB advances, rate | 0.00% | 5.55%

X
- Definition Par amount of advances made and reported by Federal Home Loan Bank (FHLBank) with fixed interest rate. + References No definition available. + Details
Name: | us-gaap_FederalHomeLoanBankAdvancesFixedRate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Interest rate of advances made and reported by Federal Home Loan Bank (FHLBank). + References No definition available. + Details
Name: | us-gaap_FederalHomeLoanBankAdvancesInterestRate
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FederalHomeLoanBankAdvancesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DebtInstrumentAxis=isba_FederalHomeLoanBankAdvancesFixedRateDue2024Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |