Note 10 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Commitment to Lend	$ 0.7	$ 0.7
Commitment to Fund Investment in SBIC Qualified Funds	1.9
Other Liabilities [Member]
Financing Receivable, Commitment to Lend	$ 0.7

Unfunded commitments to investment in Small Business Investment Company qualified funds.

No definition available.

Name:	istr_CommitmentToFundInvestmentInSBICQualifiedFunds
Namespace Prefix:	istr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant