XML 63 R53.htm IDEA: XBRL DOCUMENT
v3.19.1

Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | $ 80.0 |  | $ 83.0
Recorded Investment With No Allowance | 26.1 |  | 36.0
Recorded Investment With Allowance | 35.1 |  | 28.8
Total Recorded Investment | 61.2 |  | 64.8
Related Allowance | 7.1 |  | 6.8
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income |  |  |
Average Recorded Investment | 63.1 | $ 79.7 |
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract] |  |  |
Income Recognized | 0.0 | 0.0 |
Commercial |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 27.0 |  | 22.2
Recorded Investment With No Allowance | 10.7 |  | 8.6
Recorded Investment With Allowance | 10.6 |  | 7.7
Total Recorded Investment | 21.3 |  | 16.3
Related Allowance | 0.7 |  | 0.7
Land acquisition & development |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 10.0 |  | 10.0
Recorded Investment With No Allowance | 0.4 |  | 0.4
Recorded Investment With Allowance | 3.4 |  | 3.5
Total Recorded Investment | 3.8 |  | 3.9
Related Allowance | 0.3 |  | 0.2
Residential |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 1.5 |  | 1.1
Recorded Investment With No Allowance | 0.9 |  | 0.6
Recorded Investment With Allowance | 0.4 |  | 0.4
Total Recorded Investment | 1.3 |  | 1.0
Related Allowance | 0.0 |  | 0.1
Commercial |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 0.6 |  | 0.7
Recorded Investment With No Allowance | 0.1 |  | 0.2
Recorded Investment With Allowance | 0.0 |  | 0.0
Total Recorded Investment | 0.1 |  | 0.2
Related Allowance | 0.0 |  | 0.0
Total construction loans |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 12.1 |  | 11.8
Recorded Investment With No Allowance | 1.4 |  | 1.2
Recorded Investment With Allowance | 3.8 |  | 3.9
Total Recorded Investment | 5.2 |  | 5.1
Related Allowance | 0.3 |  | 0.3
Residential |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 8.9 |  | 8.8
Recorded Investment With No Allowance | 4.2 |  | 5.7
Recorded Investment With Allowance | 3.3 |  | 2.0
Total Recorded Investment | 7.5 |  | 7.7
Related Allowance | 0.5 |  | 0.3
Agricultural |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 2.7 |  | 12.9
Recorded Investment With No Allowance | 2.4 |  | 12.5
Recorded Investment With Allowance | 0.2 |  | 0.2
Total Recorded Investment | 2.6 |  | 12.7
Related Allowance | 0.0 |  | 0.0
Total real estate loans |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 50.7 |  | 55.7
Recorded Investment With No Allowance | 18.7 |  | 28.0
Recorded Investment With Allowance | 17.9 |  | 13.8
Total Recorded Investment | 36.6 |  | 41.8
Related Allowance | 1.5 |  | 1.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income |  |  |
Average Recorded Investment | 39.3 | 56.1 |
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract] |  |  |
Income Recognized | 0.0 | 0.0 |
Commercial |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 24.3 |  | 24.1
Recorded Investment With No Allowance | 5.8 |  | 5.5
Recorded Investment With Allowance | 14.0 |  | 14.4
Total Recorded Investment | 19.8 |  | 19.9
Related Allowance | 5.1 |  | 5.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income |  |  |
Average Recorded Investment | 19.9 | 22.1 |
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract] |  |  |
Income Recognized | 0.0 | 0.0 |
Agricultural |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |
Unpaid Total Principal Balance | 5.0 |  | 3.2
Recorded Investment With No Allowance | 1.6 |  | 2.5
Recorded Investment With Allowance | 3.2 |  | 0.6
Total Recorded Investment | 4.8 |  | 3.1
Related Allowance | 0.5 |  | $ 0.3
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income |  |  |
Average Recorded Investment | 3.9 | 1.5 |
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract] |  |  |
Income Recognized | $ 0.0 | $ 0.0 |

X
- Definition Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

X
- Definition Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

X
- Definition Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of allowance for credit losses related to recorded investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

X
- Definition Amount of investment in impaired financing receivables for which there is no related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in impaired financing receivables for which there is a related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
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_ResidentialMortgageMember
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_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: |