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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

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- 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