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Deposits (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Banking and Thrift [Abstract] |  |  |
Interest Expense, Time Deposits, Greater than $250,000 | $ 1,883 | $ 2,001 | $ 1,280

X
- Definition Interest Expense, Time Deposits, Greater than $250,000 + References No definition available. + Details
Name: | isba_InterestExpenseTimeDepositsGreaterthan250000
Namespace Prefix: | isba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_BankingAndThriftAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration