XML 82 R68.htm IDEA: XBRL DOCUMENT v2.4.1.9
Investment Securities Transfer from AFS to HTM (Details) (USD $)
In Thousands, unless otherwise specified
 0 Months Ended
Jun. 27, 2014
Schedule of Available for Sale and Held-to-Maturity Securities  
Remaining Expected Lives of Available-for-Sale Securities Transferred to Held-to-Maturity 4 years 3 months 17 days
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations  
Schedule of Available for Sale and Held-to-Maturity Securities  
Available-for-Sale Securities Transferred to Held-to-Maturity, Amortized Costs 396,640fibk_AvailableforSaleSecuritiesTransferredtoHeldtoMaturityAmortizedCosts
/ us-gaap_DebtSecurityAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-Sale Securities Transferred to Held-to-Maturity, Fair Values 388,808fibk_AvailableforSaleSecuritiesTransferredtoHeldtoMaturityFairValues
/ us-gaap_DebtSecurityAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-Sale Securities Transferred to Held-to-Maturity, Unrealized Net Gain (Loss) 7,832fibk_AvailableforSaleSecuritiesTransferredtoHeldtoMaturityUnrealizedNetGainLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember