XML 114 R92.htm IDEA: XBRL DOCUMENT v2.4.1.9
Regulatory Capital Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Total risk-based capital:    
Actual Amount $ 897,769us-gaap_Capital $ 829,443us-gaap_Capital
Actual Ratio 16.20%us-gaap_CapitalToRiskWeightedAssets 16.70%us-gaap_CapitalToRiskWeightedAssets
Adequately Capitalized Amount 444,685us-gaap_CapitalRequiredForCapitalAdequacy 396,210us-gaap_CapitalRequiredForCapitalAdequacy
Adequately Capitalized Ratio 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Tier 1 risk-based capital:    
Actual Amount 807,229us-gaap_TierOneRiskBasedCapital 739,246us-gaap_TierOneRiskBasedCapital
Actual Ratio 14.50%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets 14.90%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Adequately Capitalized Amount 222,343us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy 198,105us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
Adequately Capitalized Ratio 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Leverage capital ratio:    
Tier One Leverage Capital 807,229us-gaap_TierOneLeverageCapital 739,246us-gaap_TierOneLeverageCapital
Actual Ratio 9.60%us-gaap_TierOneLeverageCapitalToAverageAssets 10.10%us-gaap_TierOneLeverageCapitalToAverageAssets
Adequately Capitalized Amount 335,897us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy 293,414us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
Adequately Capitalized Ratio 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
FIB    
Total risk-based capital:    
Actual Amount 832,907us-gaap_Capital
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 723,955us-gaap_Capital
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Actual Ratio 15.10%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 14.70%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Adequately Capitalized Amount 442,468us-gaap_CapitalRequiredForCapitalAdequacy
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 394,038us-gaap_CapitalRequiredForCapitalAdequacy
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Adequately Capitalized Ratio 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Well Capitalized Amount 553,085us-gaap_CapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 492,548us-gaap_CapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Well Capitalized Ratio 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Tier 1 risk-based capital:    
Actual Amount 754,708us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 650,093us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Actual Ratio 13.70%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 13.20%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Adequately Capitalized Amount 221,234us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 197,019us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Adequately Capitalized Ratio 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Well Capitalized Amount 331,851us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 295,529us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Well Capitalized Ratio 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Leverage capital ratio:    
Tier One Leverage Capital 754,708us-gaap_TierOneLeverageCapital
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 650,093us-gaap_TierOneLeverageCapital
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Actual Ratio 9.20%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 8.90%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Adequately Capitalized Amount 330,006us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 292,199us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Adequately Capitalized Ratio 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Well Capitalized Amount $ 412,507us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 $ 365,248us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
Well Capitalized Ratio 5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember		 5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember