Note 4 - Other Real Estate Owned (Details Textual) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Other Real Estate Owned, Write Downs	$ 434	$ 233
Mortgage Loans in Process of Foreclosure, Amount	2,000	100
Repossessed Assets	3,374	5,218
One to Four Family Residential Property [Member]
Repossessed Assets	$ 700	$ 1,700

Represents the amount of write downs on other real estate owned during the period.

No definition available.

Name:	istr_OtherRealEstateOwnedWriteDowns
Namespace Prefix:	istr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance; includes real and personal property; equity interests in corporations, partnerships, and joint ventures; and beneficial interests in trusts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11

+ Details

Name:	us-gaap_ForeclosedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 35
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-35

+ Details

Name:	us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=istr_OneToFourFamilyResidentialPropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: