XML 63 R53.htm IDEA: XBRL DOCUMENT
v3.20.2

Loans - Schedule of Contractual Aging of Loans by Portfolio (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2020 | Dec. 31, 2019
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Total Loans |  |  | $ 8,936.2
Accrued interest reversal | $ 0.0 | $ 0.3 |
Credit card |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 1.6 | 1.6 | 2.1
Current Loans | 68.3 | 68.3 | 79.5
Nonaccrual Loans | 0.0 | 0.0 | 0.0
Total Loans | 69.9 | 69.9 | 81.6
Commercial real estate |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 9.8 | 9.8 | 7.2
Current Loans | 3,671.2 | 3,671.2 | 3,467.6
Nonaccrual Loans | 9.9 | 9.9 | 13.0
Total Loans | 3,690.9 | 3,690.9 | 3,487.8
Indirect consumer |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 5.7 | 5.7 | 10.0
Current Loans | 805.3 | 805.3 | 773.0
Nonaccrual Loans | 1.8 | 1.8 | 1.6
Total Loans | 812.8 | 812.8 | 784.6
Direct and advance lines |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.8 | 0.8 | 1.8
Current Loans | 160.8 | 160.8 | 176.7
Nonaccrual Loans | 0.5 | 0.5 | 0.5
Total Loans | 162.1 | 162.1 | 179.0
Land acquisition & development |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 2.5 | 2.5 | 1.8
Current Loans | 271.7 | 271.7 | 298.9
Nonaccrual Loans | 0.6 | 0.6 | 1.4
Total Loans | 274.8 | 274.8 | 302.1
Residential construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 2.1 | 2.1 | 2.3
Current Loans | 225.8 | 225.8 | 241.8
Nonaccrual Loans | 0.0 | 0.0 | 0.0
Total Loans | 227.9 | 227.9 | 244.1
Commercial construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.1 | 0.1 | 0.0
Current Loans | 530.2 | 530.2 | 431.0
Nonaccrual Loans | 0.5 | 0.5 | 0.5
Total Loans | 530.8 | 530.8 | 431.5
Total construction loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 4.7 | 4.7 | 4.1
Current Loans | 1,027.7 | 1,027.7 | 971.7
Nonaccrual Loans | 1.1 | 1.1 | 1.9
Total Loans | 1,033.5 | 1,033.5 | 977.7
Residential |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 4.2 | 4.2 | 6.3
Current Loans | 1,302.4 | 1,302.4 | 1,235.2
Nonaccrual Loans | 4.6 | 4.6 | 4.6
Total Loans | 1,311.2 | 1,311.2 | 1,246.1
Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.1 | 0.1 | 1.3
Current Loans | 220.0 | 220.0 | 220.1
Nonaccrual Loans | 7.6 | 7.6 | 5.2
Total Loans | 227.7 | 227.7 | 226.6
Total real estate loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 18.8 | 18.8 | 18.9
Current Loans | 6,221.3 | 6,221.3 | 5,894.6
Nonaccrual Loans | 23.2 | 23.2 | 24.7
Total Loans | 6,263.3 | 6,263.3 | 5,938.2
Total consumer loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 8.1 | 8.1 | 13.9
Current Loans | 1,034.4 | 1,034.4 | 1,029.2
Nonaccrual Loans | 2.3 | 2.3 | 2.1
Total Loans | 1,044.8 | 1,044.8 | 1,045.2
Commercial |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 15.9 | 15.9 | 9.7
Current Loans | 2,567.5 | 2,567.5 | 1,650.3
Nonaccrual Loans | 16.2 | 16.2 | 13.7
Total Loans | 2,599.6 | 2,599.6 | 1,673.7
Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 2.9 | 2.9 | 1.0
Current Loans | 268.7 | 268.7 | 275.7
Nonaccrual Loans | 3.1 | 3.1 | 2.4
Total Loans | 274.7 | 274.7 | 279.1
Other, including overdrafts |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.0
Current Loans | 4.2 | 4.2 | 0.0
Nonaccrual Loans | 0.0 | 0.0 | 0.0
Total Loans | 4.2 | 4.2 | 0.0
Loans And Leases Held For Investment [Member] |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 45.7 | 45.7 | 43.5
Current Loans | 10,096.1 | 10,096.1 | 8,849.8
Nonaccrual Loans | 44.8 | 44.8 | 42.9
Total Loans | 10,186.6 | 10,186.6 | 8,936.2
30 to 59 Days Past Due | Credit card |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.7 | 0.7 | 0.8
30 to 59 Days Past Due | Commercial real estate |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 5.6 | 5.6 | 5.5
30 to 59 Days Past Due | Indirect consumer |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 4.0 | 4.0 | 7.6
30 to 59 Days Past Due | Direct and advance lines |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.5 | 0.5 | 1.2
30 to 59 Days Past Due | Land acquisition & development |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 1.4 | 1.4 | 0.7
30 to 59 Days Past Due | Residential construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.4 | 0.4 | 1.5
30 to 59 Days Past Due | Commercial construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.1 | 0.1 | 0.0
30 to 59 Days Past Due | Total construction loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 1.9 | 1.9 | 2.2
30 to 59 Days Past Due | Residential |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 1.7 | 1.7 | 3.8
30 to 59 Days Past Due | Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.8
30 to 59 Days Past Due | Total real estate loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 9.2 | 9.2 | 12.3
30 to 59 Days Past Due | Total consumer loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 5.2 | 5.2 | 9.6
30 to 59 Days Past Due | Commercial |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 5.6 | 5.6 | 4.8
30 to 59 Days Past Due | Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 1.8 | 1.8 | 0.9
30 to 59 Days Past Due | Other, including overdrafts |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.0
30 to 59 Days Past Due | Loans And Leases Held For Investment [Member] |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 21.8 | 21.8 | 27.6
60 to 89 Days Past Due | Credit card |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.3 | 0.3 | 0.5
60 to 89 Days Past Due | Commercial real estate |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.6 | 0.6 | 1.1
60 to 89 Days Past Due | Indirect consumer |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 1.6 | 1.6 | 1.9
60 to 89 Days Past Due | Direct and advance lines |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.2 | 0.2 | 0.5
60 to 89 Days Past Due | Land acquisition & development |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.6 | 0.6 | 0.8
60 to 89 Days Past Due | Residential construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.2 | 0.2 | 0.8
60 to 89 Days Past Due | Commercial construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.0
60 to 89 Days Past Due | Total construction loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.8 | 0.8 | 1.6
60 to 89 Days Past Due | Residential |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 2.3 | 2.3 | 1.4
60 to 89 Days Past Due | Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.1 | 0.1 | 0.5
60 to 89 Days Past Due | Total real estate loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 3.8 | 3.8 | 4.6
60 to 89 Days Past Due | Total consumer loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 2.1 | 2.1 | 2.9
60 to 89 Days Past Due | Commercial |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 8.2 | 8.2 | 2.6
60 to 89 Days Past Due | Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.2 | 0.2 | 0.1
60 to 89 Days Past Due | Other, including overdrafts |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.0
60 to 89 Days Past Due | Loans And Leases Held For Investment [Member] |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 14.3 | 14.3 | 10.2
Equal to or Greater than 90 Days Past Due | Credit card |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.6 | 0.6 | 0.8
Equal to or Greater than 90 Days Past Due | Commercial real estate |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 3.6 | 3.6 | 0.6
Equal to or Greater than 90 Days Past Due | Indirect consumer |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.1 | 0.1 | 0.5
Equal to or Greater than 90 Days Past Due | Direct and advance lines |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.1 | 0.1 | 0.1
Equal to or Greater than 90 Days Past Due | Land acquisition & development |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.5 | 0.5 | 0.3
Equal to or Greater than 90 Days Past Due | Residential construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 1.5 | 1.5 | 0.0
Equal to or Greater than 90 Days Past Due | Commercial construction |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.0
Equal to or Greater than 90 Days Past Due | Total construction loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 2.0 | 2.0 | 0.3
Equal to or Greater than 90 Days Past Due | Residential |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.2 | 0.2 | 1.1
Equal to or Greater than 90 Days Past Due | Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.0
Equal to or Greater than 90 Days Past Due | Total real estate loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 5.8 | 5.8 | 2.0
Equal to or Greater than 90 Days Past Due | Total consumer loans |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.8 | 0.8 | 1.4
Equal to or Greater than 90 Days Past Due | Commercial |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 2.1 | 2.1 | 2.3
Equal to or Greater than 90 Days Past Due | Agricultural |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.9 | 0.9 | 0.0
Equal to or Greater than 90 Days Past Due | Other, including overdrafts |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | 0.0 | 0.0 | 0.0
Equal to or Greater than 90 Days Past Due | Loans And Leases Held For Investment [Member] |  |  |
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items] |  |  |
Past Due | $ 9.6 | $ 9.6 | $ 5.7

X
- Definition Gross carrying amount of all categories of loans and leases held in the portfolio, excluding loans held for sale. + References No definition available. + Details
Name: | fibk_LoansAndLeasesReceivableHeldForInvestment
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of writeoff of accrued interest on financing receivable recognized by reversing interest income. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 3D -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL120267969-210447 + Details
Name: | us-gaap_FinancingReceivableAccruedInterestWriteoff
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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_FinancingReceivableModificationsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of financing receivable not past due. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953401-111524 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 80 -URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919253-210447 + Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable on nonaccrual status. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 16 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447 + Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable past due. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 80 -URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448 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=121599337&loc=SL82919253-210447 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953401-111524 + Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentPastDue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CreditCardReceivablesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_ConsumerIndirectFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_LandAcquisitionAndDevelopmentConstructionFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_ResidentialRealEstateConstructionFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_CommercialRealEstateConstructionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_RealEstateConstructionFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_AgriculturalRealEstateFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_RealEstateFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_AgriculturalFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_OtherFinancingReceivableIncludingOverdraftsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_LoansAndLeasesHeldForInvestmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |