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Borrowed Funds (Borrowed funds obligations) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Debt [Line Items]
Borrowed funds	$ 275,999	$ 340,299
Borrowed funds, Rate	2.07%	1.95%
Securities Sold under Agreements to Repurchase [Member]
Schedule of Debt [Line Items]
Borrowed funds	$ 30,999	$ 40,299
Borrowed funds, Rate	0.09%	0.11%
Federal Home Loan Bank Advances [Member]
Schedule of Debt [Line Items]
Borrowed funds	$ 245,000	$ 300,000
Borrowed funds, Rate	2.32%	2.20%

X

- Definition

[Line Items] for Schedule of Debt [Table]

+ References

No definition available.

+ Details

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Namespace Prefix:	isba_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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

X

- Details

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Namespace Prefix:
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Period Type: