XML 91 R82.htm IDEA: XBRL DOCUMENT
v3.22.2.2

Other Borrowed Funds - Narrative (Details) $ in Millions | Sep. 30, 2022 USD ($)
--- | ---
Short-term Debt, Percentage Bearing Variable Interest Rate | 329.00%
Short-term Debt, Percentage Bearing Fixed Interest Rate | 320.00%
Debt Instrument, Unused Borrowing Capacity, Amount | $ 3,962.4
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged | 4,588.0
Variable Rate [Member] |
Other Borrowings | 550.0
Fixed Rate [Member] |
Other Borrowings | $ 75.0

X
- Definition Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of the collateral pledged in support of federal home loan bank advances outstanding. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=123599511&loc=d3e64711-112823 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822 + Details
Name: | us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The carrying amount as of the balance sheet date for the aggregate of other miscellaneous borrowings owed by the reporting entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13,16) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
Name: | us-gaap_OtherBorrowings
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The portion of the carrying amount of short-term borrowings outstanding as of the balance sheet date which accrues interest at a set, unchanging rate. + References No definition available. + Details
Name: | us-gaap_ShortTermDebtPercentageBearingFixedInterestRate
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The portion of the carrying amount of short-term borrowings outstanding as of the balance sheet date, which accrues interest at a rate subject to change from time to time. + References No definition available. + Details
Name: | us-gaap_ShortTermDebtPercentageBearingVariableInterestRate
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | fibk_InterestRateTypeAxis=fibk_VariableRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_InterestRateTypeAxis=fibk_FixedRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |