Commitments and Contingencies - Additional Information (Detail) $ in Millions	Sep. 30, 2016 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Commitment to fund investment in SBIC qualified funds	$ 0.9

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