XML 52 R41.htm IDEA: XBRL DOCUMENT
v3.25.2

Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2025 | Jun. 30, 2025 | Dec. 31, 2024
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | $ 53.7 | $ 133.4
Gross Unrealized Losses, Less than 12 Months |  | (0.6) | (2.7)
Fair Value, 12 Months or Longer |  | 3,877.3 | 4,047.2
Gross Unrealized Losses, 12 Months or Longer |  | (306.3) | (417.4)
Fair Value, Total |  | 3,931.0 | 4,180.6
Gross Unrealized Losses, Total |  | (306.9) | (420.1)
Allowance for Credit Losses |  | 0.0 |
Gross Unrealized Gains |  | 4.6 | 2.4
Gross Unrealized Losses |  | (306.9) | (420.1)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 4,742.2 | 5,057.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 5,044.5 | 5,474.8
Investment securities classified as held to maturity |  | 2,570.7 | 2,688.4
Allowance for Credit Losses |  | (0.7) | (0.9)
Net Amortized Cost |  | 2,570.0 | 2,687.5
Gross Unrealized Gains |  | 0.9 | 0.2
Gross Unrealized Losses |  | (254.6) | (330.0)
Investment debt securities held-to-maturity |  | 2,317.0 | 2,358.6
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss | $ 15.9 |  |
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain | $ (8.0) |  |
US Treasury Notes Securities |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 0.0 |
Gross Unrealized Losses, Less than 12 Months |  | 0.0 |
Fair Value, 12 Months or Longer |  | 233.9 |
Gross Unrealized Losses, 12 Months or Longer |  | (11.8) |
Fair Value, Total |  | 233.9 |
Gross Unrealized Losses, Total |  | (11.8) |
U.S. Treasury notes |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  |  | 0.0
Gross Unrealized Losses, Less than 12 Months |  |  | 0.0
Fair Value, 12 Months or Longer |  |  | 226.8
Gross Unrealized Losses, 12 Months or Longer |  |  | (18.1)
Fair Value, Total |  |  | 226.8
Gross Unrealized Losses, Total |  |  | (18.1)
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (11.8) | (18.1)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 233.9 | 226.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 245.7 | 245.0
Investment securities classified as held to maturity |  | 100.0 | 99.8
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (0.1) | (0.4)
Investment debt securities held-to-maturity |  | 99.9 | 99.4
State, county, and municipal securities |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 0.0 | 0.0
Gross Unrealized Losses, Less than 12 Months |  | 0.0 | 0.0
Fair Value, 12 Months or Longer |  | 211.6 | 210.1
Gross Unrealized Losses, 12 Months or Longer |  | (35.8) | (40.0)
Fair Value, Total |  | 211.6 | 210.1
Gross Unrealized Losses, Total |  | (35.8) | (40.0)
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (35.8) | (40.0)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 213.4 | 212.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 249.2 | 252.0
Investment securities classified as held to maturity |  | 175.8 | 176.3
Gross Unrealized Gains |  | 0.2 | 0.2
Gross Unrealized Losses |  | (23.5) | (26.7)
Investment debt securities held-to-maturity |  | 152.5 | 149.8
Obligations of U.S. government agencies |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 1.0 | 3.6
Gross Unrealized Losses, Less than 12 Months |  | 0.0 | 0.0
Fair Value, 12 Months or Longer |  | 200.7 | 206.2
Gross Unrealized Losses, 12 Months or Longer |  | (6.5) | (10.8)
Fair Value, Total |  | 201.7 | 209.8
Gross Unrealized Losses, Total |  | (6.5) | (10.8)
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (6.5) | (10.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 206.7 | 213.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 213.2 | 224.1
Investment securities classified as held to maturity |  | 466.8 | 470.1
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (45.2) | (59.9)
Investment debt securities held-to-maturity |  | 421.6 | 410.2
Private mortgage-backed securities |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 0.0 | 0.0
Gross Unrealized Losses, Less than 12 Months |  | 0.0 | 0.0
Fair Value, 12 Months or Longer |  | 182.2 | 190.4
Gross Unrealized Losses, 12 Months or Longer |  | (23.2) | (27.6)
Fair Value, Total |  | 182.2 | 190.4
Gross Unrealized Losses, Total |  | (23.2) | (27.6)
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (23.2) | (27.6)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 182.3 | 190.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 205.5 | 218.1
Collateralized Loan Obligations |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  |  | 10.0
Gross Unrealized Losses, Less than 12 Months |  |  | 0.0
Fair Value, 12 Months or Longer |  |  | 0.0
Gross Unrealized Losses, 12 Months or Longer |  |  | 0.0
Fair Value, Total |  |  | 10.0
Gross Unrealized Losses, Total |  |  | 0.0
Gross Unrealized Gains |  | 1.0 | 1.3
Gross Unrealized Losses |  | 0.0 | 0.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 626.8 | 772.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 625.8 | 770.7
Corporate Debt Securities |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 13.0 | 27.5
Gross Unrealized Losses, Less than 12 Months |  | 0.0 | (0.5)
Fair Value, 12 Months or Longer |  | 192.1 | 205.5
Gross Unrealized Losses, 12 Months or Longer |  | (12.0) | (16.2)
Fair Value, Total |  | 205.1 | 233.0
Gross Unrealized Losses, Total |  | (12.0) | (16.7)
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (12.0) | (16.7)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 227.0 | 232.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 239.0 | 249.6
Investment securities classified as held to maturity |  | 40.0 | 57.0
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (1.8) | (2.8)
Investment debt securities held-to-maturity |  | 38.2 | 54.2
Commercial Mortgage-Backed Securities |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 0.0 | 38.1
Gross Unrealized Losses, Less than 12 Months |  | 0.0 | (0.4)
Fair Value, 12 Months or Longer |  | 898.0 | 934.9
Gross Unrealized Losses, 12 Months or Longer |  | (49.6) | (74.1)
Fair Value, Total |  | 898.0 | 973.0
Gross Unrealized Losses, Total |  | (49.6) | (74.5)
Gross Unrealized Gains |  | 1.0 | 0.1
Gross Unrealized Losses |  | (49.6) | (74.5)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 947.1 | 982.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 995.7 | 1,057.2
Investment securities classified as held to maturity |  | 369.7 | 374.9
Gross Unrealized Gains |  | 0.0 | 0.0
Gross Unrealized Losses |  | (24.8) | (33.4)
Investment debt securities held-to-maturity |  | 344.9 | 341.5
Residential Mortgage Backed Securities |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 32.9 | 33.7
Gross Unrealized Losses, Less than 12 Months |  | (0.6) | (1.5)
Fair Value, 12 Months or Longer |  | 1,022.3 | 1,086.2
Gross Unrealized Losses, 12 Months or Longer |  | (87.4) | (124.3)
Fair Value, Total |  | 1,055.2 | 1,119.9
Gross Unrealized Losses, Total |  | (88.0) | (125.8)
Gross Unrealized Gains |  | 1.2 | 0.4
Gross Unrealized Losses |  | (88.0) | (125.8)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 1,111.4 | 1,175.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 1,198.2 | 1,301.2
Investment securities classified as held to maturity |  | 1,013.4 | 1,082.5
Gross Unrealized Gains |  | 0.2 | 0.0
Gross Unrealized Losses |  | (105.1) | (140.7)
Investment debt securities held-to-maturity |  | 908.5 | 941.8
Collateralized Mortgage Obligations |  |  |
Available-for-Sale: |  |  |
Fair Value, Less than 12 Months |  | 6.8 | 20.5
Gross Unrealized Losses, Less than 12 Months |  | 0.0 | (0.3)
Fair Value, 12 Months or Longer |  | 936.5 | 987.1
Gross Unrealized Losses, 12 Months or Longer |  | (80.0) | (106.3)
Fair Value, Total |  | 943.3 | 1,007.6
Gross Unrealized Losses, Total |  | (80.0) | (106.6)
Gross Unrealized Gains |  | 1.4 | 0.6
Gross Unrealized Losses |  | (80.0) | (106.6)
Debt Securities, Available-for-Sale, Excluding Accrued Interest |  | 993.6 | 1,050.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss |  | 1,072.2 | 1,156.9
Investment securities classified as held to maturity |  | 405.0 | 427.8
Gross Unrealized Gains |  | 0.5 | 0.0
Gross Unrealized Losses |  | (54.1) | (66.1)
Investment debt securities held-to-maturity |  | 351.4 | $ 361.7
US agency, residential and commercial MBS and CMOs |  |  |
Available-for-Sale: |  |  |
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain |  | (6.9) |
State, county & municipal, obligations of US govt agencies and CLOs |  |  |
Available-for-Sale: |  |  |
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss |  | $ 14.8 |

X
- Definition Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain + References No definition available. + Details
Name: | fibk_DebtSecuritiesHeldToMaturityTransferUnamortizedGain
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss + References No definition available. + Details
Name: | fibk_DebtSecuritiesHeldToMaturityTransferUnamortizedLoss
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount excluding accrued interest, of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479130/326-30-45-1 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479130/326-30-45-1 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479130/326-30-45-1 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (aaa) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479344/326-20-45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-13 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-13 + Details
Name: | us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481830/320-10-45-1 + Details
Name: | us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Publisher FASB -URI https://asc.fasb.org/1943274/2147481962/310-10-50-7A Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479344/326-20-45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-14 + Details
Name: | us-gaap_HeldToMaturitySecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5A + Details
Name: | us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5A + Details
Name: | us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5A Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
Name: | us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryNotesSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedLoanObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fibk_USAgencyResidentialAndCommercialMBSAndCMOsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fibk_StateCountyMunicipalObligationsOfUSGovtAgenciesAndCLOsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |