XML 51 R41.htm IDEA: XBRL DOCUMENT
v3.20.2

Acquisitions - Schedule of Acquired Loans not Deemed to Have Credit Impairment (Details) - Idaho Independent Bank $ in Millions | Apr. 08, 2019 USD ($)
--- | ---
Business Acquisition [Line Items] |
Contractually required principal and interest payments | $ 398.7
Contractual cash flows not expected to be collected | 15.2
Fair value at acquisition | $ 321.5

X
- Definition Loans Acquired in Transfer, Contractually Required Payments Receivable at Acquisition + References No definition available. + Details
Name: | fibk_LoansAcquiredinTransferContractuallyRequiredPaymentsReceivableatAcquisition
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Loans Acquired in Transfer, Nonaccretable Difference + References No definition available. + Details
Name: | fibk_LoansAcquiredinTransferNonaccretableDifference
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Loans Acquired in Transfer at Acquisition Fair Value + References No definition available. + Details
Name: | fibk_LoansAcquiredinTransferatAcquisitionFairValue
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_IdahoIndependentBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |