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Commitments and Contingencies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Loss Contingencies [Line Items]    
Commitments to extend credit 121.8us-gaap_UnusedCommitmentsToExtendCredit $ 91.5us-gaap_UnusedCommitmentsToExtendCredit
Average reserve balance maintained with Federal Reserve Bank 10.5istr_AverageReserveBalanceMaintainedWithFederalReserveBank $ 12.9istr_AverageReserveBalanceMaintainedWithFederalReserveBank
Letter of credit [Member]    
Loss Contingencies [Line Items]    
Letters of credit expiration period 1 year