XML 86 R77.htm IDEA: XBRL DOCUMENT
v3.23.1

Commitments and Contingencies (Details) - USD ($) $ in Millions | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Loss Contingencies [Line Items] |  |
Shares outstanding (in shares) | 104,382,018 | 104,442,023
Class A Common Stock |  |
Loss Contingencies [Line Items] |  |
Shares outstanding (in shares) |  | 104,442,023
Mortgage Loans Held For Sale [Member] |  |
Loss Contingencies [Line Items] |  |
Mortgage loans with recourse provision in effect | $ 0.8 |
Construction Contracts |  |
Loss Contingencies [Line Items] |  |
Commitments under construction contracts | $ 6.5 |

X
- Definition Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=126973232&loc=d3e21463-112644 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_CommonStockSharesOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=124440162&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsCurrentCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=123406679&loc=d3e25336-109308 + Details
Name: | us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=us-gaap_ConstructionContractsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |