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Borrowed Funds (Short-term borrowings) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Securities Sold under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Maximum Month End Balance	$ 38,967	$ 58,088
Average Balance	$ 35,995	$ 57,505
Weighted Average Interest Rate During the Period	0.10%	0.13%
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Maximum Month End Balance	$ 10,200	$ 5,200
Average Balance	$ 4,460	$ 863
Weighted Average Interest Rate During the Period	1.66%	0.96%

X

- Definition

Short term debt weighted average interest rates.

+ References

No definition available.

+ Details

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Namespace Prefix:	isba_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period.

+ References

No definition available.

+ Details

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

X

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, maximum month-end borrowings during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Details

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Namespace Prefix:
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Period Type:

X

- Details

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