XML 46 R32.htm IDEA: XBRL DOCUMENT
v3.21.2

Investment Securities - Narrative (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Mar. 31, 2021 USD ($) | Sep. 30, 2021 USD ($) Investments | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Investment securities classified as held to maturity |  | $ 1,618.1 |  | $ 51.6
Investment securities in an unrealized loss position (securities) | Investments |  | 278 |  | 181
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 0.0 |  | $ 0.0
Municipal Debt Securities, at Carrying Value |  | $ 56.9 |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Debt Securities |  | 71.90% |  |
Debt Securities, Available-for-sale, Amortized Cost |  | $ 4,408.8 |  | 3,932.7
Debt Securities, Available-for-sale, Realized Gain |  | 0.0 | $ 0.0 |
Debt Securities, Held-to-maturity, Sold, Realized Gain (Loss) | $ 0.0 |  |  |
Debt Securities, Available-for-sale, Realized Loss |  | 0.0 | $ 0.0 |
State, county and municipal securities |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Investment securities classified as held to maturity |  | 69.7 |  | 46.6
Debt Securities, Available-for-sale, Amortized Cost |  | 444.9 |  | $ 462.1
813940 Political Organizations [Member] |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Municipal Debt Securities, at Carrying Value |  | 40.9 |  |
Residential Mortgage Backed Securities |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 540.2 |  |
Callable Within One Year |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized cost of investment securities callable after one year but within five years |  | 212.3 |  |
Fair value of investment securities callable after one year but within five years |  | 209.9 |  |
Callable structured notes amortized costs |  | $ 0.0 |  |

X
- Definition Available-for-Sale and Held-to-Maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions + References No definition available. + Details
Name: | fibk_AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Namespace Prefix: | fibk_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Investment Securities,Callable Structured, Amortized Cost Basis + References No definition available. + Details
Name: | fibk_InvestmentSecuritiesCallableStructuredAmortizedCostBasis
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Investment Securities, Primarily Classified as Available-for-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis + References No definition available. + Details
Name: | fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Investment Securities, Primarily Classified as Available-for-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value + References No definition available. + Details
Name: | fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveFairValue
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost 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 -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount 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 -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922900-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922900-210455 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of realized gain 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 -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27357-111563 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleRealizedGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of realized 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 -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27357-111563 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The percentage of the total value of investments that are debt securities for a given category of investments for which net asset value per share is calculated. + References No definition available. + Details
Name: | us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerSharePercentDebtSecurities
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
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://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919253-210447 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953401-111524 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 + Details
Name: | us-gaap_HeldToMaturitySecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of gain (loss) on sale of investment in debt security measured at amortized cost (held-to-maturity), including other-than-temporary impairment (OTTI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27405-111563 + Details
Name: | us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Bonds or similar securities issued by state, city, or local governments or the agencies operated by state, city, or local governments which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months. + References No definition available. + Details
Name: | us-gaap_MunicipalDebtSecuritiesAtCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=naics_ZZ813940
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_InvestmentTypeAxis=fibk_CallableWithinOneYearMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |