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Real Estate Owned - Additional Information (Detail) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Real Estate Properties [Line Items]      
Real estate owned related to the acquisition $ 2,735,000us-gaap_ForeclosedAssets $ 3,515,000us-gaap_ForeclosedAssets $ 2,276,000us-gaap_ForeclosedAssets
FCB [Member]      
Real Estate Properties [Line Items]      
Real estate owned related to the acquisition 1,300,000us-gaap_ForeclosedAssets
/ us-gaap_BusinessAcquisitionAxis
= istr_FirstCommunityBankMember		 1,600,000us-gaap_ForeclosedAssets
/ us-gaap_BusinessAcquisitionAxis
= istr_FirstCommunityBankMember		  
South Louisiana Business Bank [Member]      
Real Estate Properties [Line Items]      
Real estate owned related to the acquisition $ 0us-gaap_ForeclosedAssets
/ us-gaap_BusinessAcquisitionAxis
= istr_SouthLouisianaBusinessBankAcquisitionMember		 $ 0us-gaap_ForeclosedAssets
/ us-gaap_BusinessAcquisitionAxis
= istr_SouthLouisianaBusinessBankAcquisitionMember