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Borrowed Funds (Borrowed funds obligations) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Debt [Line Items]
Borrowed funds	$ 340,299	$ 344,878
Borrowed funds, Rate	1.95%	1.65%
Securities Sold under Agreements to Repurchase [Member]
Schedule of Debt [Line Items]
Borrowed funds	$ 40,299	$ 54,878
Borrowed funds, Rate	0.11%	0.12%
Federal Home Loan Bank Advances [Member]
Schedule of Debt [Line Items]
Borrowed funds	$ 300,000	$ 290,000
Borrowed funds, Rate	2.20%	1.94%

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:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: