XML 60 R51.htm IDEA: XBRL DOCUMENT
v3.22.2.2

Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) | 3 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2022 | Dec. 31, 2021 | Jun. 30, 2022 | Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2020
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | $ 7,448,200,000 | $ 4,859,300,000 |  |  |  |
Gross Unrealized Gains | 200,000 | 20,600,000 |  |  |  |
Gross Unrealized Losses | (665,000,000.0) | (59,400,000) |  |  |  |
Estimated Fair Value | 6,783,400,000 | 4,820,500,000 |  |  |  |
Held-to-Maturity: |  |  |  |  |  |
Amortized Cost | 3,487,600,000 | 1,687,600,000 |  |  |  |
Allowance for Credit Losses | (1,900,000) | 0 | $ (1,600,000) | $ 0 | $ 0 | $ 0
Net Carrying Amount | 3,485,700,000 | 1,687,600,000 |  |  |  |
Gross Unrealized Gains | 200,000 | 15,600,000 |  |  |  |
Gross Unrealized Losses | (422,400,000) | (35,700,000) |  |  |  |
Estimated Fair Value | 3,063,500,000 | 1,667,500,000 |  |  |  |
U.S. Treasury notes |  |  |  |  |  |
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | 373,500,000 | 697,600,000 |  |  |  |
Gross Unrealized Gains | 0 | 0 |  |  |  |
Gross Unrealized Losses | (36,700,000) | (12,900,000) |  |  |  |
Estimated Fair Value | 336,800,000 | 684,700,000 |  |  |  |
Held-to-Maturity: |  |  |  |  |  |
Amortized Cost | 396,100,000 |  |  |  |  |
Allowance for Credit Losses | 0 |  |  |  |  |
Net Carrying Amount | 396,100,000 |  |  |  |  |
Gross Unrealized Gains | 0 |  |  |  |  |
Gross Unrealized Losses | (10,600,000) |  |  |  |  |
Estimated Fair Value | 385,500,000 |  |  |  |  |
State, county, and municipal securities |  |  |  |  |  |
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | 317,800,000 | 434,700,000 |  |  |  |
Gross Unrealized Gains | 0 | 2,100,000 |  |  |  |
Gross Unrealized Losses | (57,300,000) | (9,300,000) |  |  |  |
Estimated Fair Value | 260,500,000 | 427,500,000 |  |  |  |
Held-to-Maturity: |  |  |  |  |  |
Amortized Cost | 184,000,000.0 | 67,600,000 |  |  |  |
Allowance for Credit Losses | (100,000) | 0 |  |  |  |
Net Carrying Amount | 183,900,000 | 67,600,000 |  |  |  |
Gross Unrealized Gains | 200,000 | 2,000,000.0 |  |  |  |
Gross Unrealized Losses | (33,200,000) | (400,000) |  |  |  |
Estimated Fair Value | 150,900,000 | 69,200,000 |  |  |  |
Obligations of U.S. government agencies |  |  |  |  |  |
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | 216,300,000 | 356,000,000.0 |  |  |  |
Gross Unrealized Gains | 0 | 100,000 |  |  |  |
Gross Unrealized Losses | (17,300,000) | (9,200,000) |  |  |  |
Estimated Fair Value | 199,000,000.0 | 346,900,000 |  |  |  |
Held-to-Maturity: |  |  |  |  |  |
Amortized Cost | 350,800,000 |  |  |  |  |
Allowance for Credit Losses | 0 |  |  |  |  |
Net Carrying Amount | 350,800,000 |  |  |  |  |
Gross Unrealized Gains | 0 |  |  |  |  |
Gross Unrealized Losses | (51,200,000) |  |  |  |  |
Estimated Fair Value | 299,600,000 |  |  |  |  |
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations |  |  |  |  |  |
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | 4,852,200,000 | 2,027,300,000 |  |  |  |
Gross Unrealized Gains | 200,000 | 14,100,000 |  |  |  |
Gross Unrealized Losses | (442,700,000) | (23,300,000) |  |  |  |
Estimated Fair Value | 4,409,700,000 | 2,018,100,000 |  |  |  |
Held-to-Maturity: |  |  |  |  |  |
Amortized Cost | 2,476,600,000 | 1,609,000,000 |  |  |  |
Allowance for Credit Losses | 0 | 0 |  |  |  |
Net Carrying Amount | 2,476,600,000 | 1,609,000,000 |  |  |  |
Gross Unrealized Gains | 0 | 13,200,000 |  |  |  |
Gross Unrealized Losses | (320,000,000.0) | (35,300,000) |  |  |  |
Estimated Fair Value | 2,156,600,000 | 1,586,900,000 |  |  |  |
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain | 15,200,000 | 20,100,000 |  |  |  |
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss | (20,800,000) |  |  |  |  |
Private mortgage-backed securities |  |  |  |  |  |
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | 270,300,000 | 174,400,000 |  |  |  |
Gross Unrealized Gains | 0 | 100,000 |  |  |  |
Gross Unrealized Losses | (37,100,000) | (1,100,000) |  |  |  |
Estimated Fair Value | 233,200,000 | 173,400,000 |  |  |  |
Collateralized Loan Obligations |  |  |  |  |  |
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | 1,145,500,000 | 898,200,000 |  |  |  |
Gross Unrealized Gains | 0 | 1,200,000 |  |  |  |
Gross Unrealized Losses | (40,400,000) | 0 |  |  |  |
Estimated Fair Value | 1,105,100,000 | 899,400,000 |  |  |  |
Corporate Debt Securities |  |  |  |  |  |
Available-for-Sale: |  |  |  |  |  |
Amortized Cost | 272,600,000 | 271,100,000 |  |  |  |
Gross Unrealized Gains | 0 | 3,000,000.0 |  |  |  |
Gross Unrealized Losses | (33,500,000) | (3,600,000) |  |  |  |
Estimated Fair Value | 239,100,000 | 270,500,000 |  |  |  |
Held-to-Maturity: |  |  |  |  |  |
Amortized Cost | 80,100,000 | 11,000,000.0 |  |  |  |
Allowance for Credit Losses | (1,800,000) | 0 |  |  |  |
Net Carrying Amount | 78,300,000 | 11,000,000.0 |  |  |  |
Gross Unrealized Gains | 0 | 400,000 |  |  |  |
Gross Unrealized Losses | (7,400,000) | 0 |  |  |  |
Estimated Fair Value | $ 70,900,000 | $ 11,400,000 |  |  |  |

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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix: | us-gaap_
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
- 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 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 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 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesDebtSecurities
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 -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -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 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 + 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://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 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/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563 + 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/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563 + Details
Name: | us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

X
- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563 + 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_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_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
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: |