XML 87 R78.htm IDEA: XBRL DOCUMENT
v3.23.1

Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2023 | Mar. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2021
Entity Information [Line Items] |  |  |  |
Off-Balance Sheet, Credit Loss, Liability | $ 17.8 | $ 6.2 | $ 16.2 | $ 3.8
Provision for credit loss expense | (12.2) | (57.3) |  |
Credit Extension Commitments | 4,976.5 |  | 5,173.3 |
Unused Credit Card Lines |  |  |  |
Entity Information [Line Items] |  |  |  |
Credit Extension Commitments | 819.8 |  | 827.6 |
Standby Letter of Credit |  |  |  |
Entity Information [Line Items] |  |  |  |
Credit Extension Commitments | 83.0 |  | $ 93.8 |
Cumulative effect, period of adoption, adjusted balance |  |  |  |
Entity Information [Line Items] |  |  |  |
Provision for credit loss expense | $ 1.6 | $ 2.4 |  |

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: | dei_EntityInformationLineItems
Namespace Prefix: | dei_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Credit Extension Commitments + References No definition available. + Details
Name: | fibk_CreditExtensionCommitments
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 45 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=121611286&loc=d3e11927-110247 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82919202-210446 + Details
Name: | us-gaap_OffBalanceSheetCreditLossLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. + 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 (b) -URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
Name: | us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=fibk_UnusedCreditCardLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_FinancialStandbyLetterOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |