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Borrowed Funds - Maturity and Weighted Average Interest Rates of FHLB Advances (Details) - Fixed rate due 2025 - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Federal Home Loan Bank, Advances [Line Items]
FHLB fixed rate advances	$ 0	$ 30,000
FHLB advances, rate	0.00%	4.52%

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_FederalHomeLoanBankAdvancesFixedRateDue2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: