Note 10 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Commitment to Lend	$ 0.2	$ 0.2
Commitment to Fund Investment in SBIC Qualified Funds	1.0
Other Liabilities [Member]
Financing Receivable, Commitment to Lend	$ 0.2	$ 0.2

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