XML 81 R99.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Entity Information |  |
Credit extension commitments | $ 2,807.8 | $ 2,620.4
Unused credit card lines |  | 1,058.5
Unused Credit Card Lines |  |
Entity Information |  |
Credit extension commitments | 764.1 | 726.7
Unused credit card lines | 1,212.7 |
Standby Letters of Credit |  |
Entity Information |  |
Credit extension commitments | $ 42.7 | $ 46.7

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 Credit Extension Commitments, Beyond One Year + References No definition available. + Details
Name: | fibk_CreditExtensionCommitmentsBeyondOneYear
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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: |