Note 1 - Description of Plan 1 (Details Textual) - EBP 27-1560715 001 [Member]	12 Months Ended
Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
EBP Age Requirement	21
EBP, Participant Contribution, Automatic, Deferral Rate	5.00%
EBP, Annual Deferral Increase Percent	1.00%
EBP, Automatic Enrollment, Max Percentage Per Employee	10.00%
EBP, Participant Contribution, Pretax, Maximum Annual Compensation, Amount	$ 23,500	$ 23,000
EBP, Minimum Age for Voluntary Catch-up Contributions	50
EBP, Employer Contribution, Matching Percentage	100.00%
EBP, Employer Contribution, Participant Compensation Matched, Percentage	4.00%
EBP, Vesting Percentage Per Year	20.00%
EBP, Award Vesting Period (Year)	5 years
EBP, Award Vesting Percentage After Six Years of Services	100.00%
EBP, Maximum of Vested Value of Participant's Accounts Required to Make Lump Sum Cash Payments	$ 1,000
EBP, Forfeited Nonvested Account, Decrease from Employer Contribution	2,019
EBP, Forfeited Nonvested Account, Decrease from Administrative Expense	2,057
EBP, Forfeited Nonvested Account	4,681	$ 7,148
EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount	1,000
EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount	$ 50,000
EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage	50.00%
Minimum [Member]
EBP, Note Receivable From Participant, Term (Year)	1 year
EBP, Asset Held for Investment, Participant Loan, Interest Rate	4.25%
Maximum [Member]
EBP, Note Receivable From Participant, Term (Year)	5 years
EBP, Asset Held for Investment, Participant Loan, Interest Rate	9.50%

The age requirement for the employee benefit plan.

No definition available.

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Namespace Prefix:	istr_
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Balance Type:	na
Period Type:	instant

- Definition

The annual deferral increase percent.

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum percentage of automatic enrollment per employee for employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage vested after six years of service.

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The vesting period for the employee benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum of Vested Value of Participant's Accounts Required to Make Lump Sum Cash Payments.

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The minimum age of participant to make voluntary catch-up contributions to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The term length for the notes receivable from participant.

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The vesting percent per year for the employee benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	istr_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanEmployerContributionMatchingPercentage
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of participant compensation eligible for employer matching contribution for participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of plan asset, in forfeited nonvested account, no longer allocated to participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of decrease in forfeited nonvested account of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) from administrative expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-04(5)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

+ Details

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

- Definition

Amount of decrease in forfeited nonvested account of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) from employer contribution.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 962
-SubTopic 325
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478873/962-325-55-17

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-04(5)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

+ Details

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

- Definition

Amount of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow to total participant account balance.

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of minimum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of eligible compensation contributed for participant automatically enrolled in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of maximum pretax annual compensation that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), is permitted to contribute.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest rate on participant loan in schedule of asset held for investment of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Employee Retirement Income Security Act of 1974
-Number 2520
-Section 103
-Subsection 10
-Paragraph b
-Subparagraph (1)(i)(D)
-Publisher DOL-EBSA

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-05(a)(Schedule I))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

+ Details

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Balance Type:	na
Period Type:	duration

- Details

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- Details

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- Details

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