XML 66 R56.htm IDEA: XBRL DOCUMENT
v3.20.2

Derivatives and Hedging Activities - Schedule of Notional Amounts of Outstanding Derivative Positions (Details) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) | May 01, 2020 USD ($) | Dec. 31, 2019 USD ($)
Derivative [Line Items] |  |  |
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net | $ 0.0 |  |
Loss on Fair Value Hedge Ineffectiveness | $ 0.0 |  |
Document Period End Date | Sep. 30, 2020 |  |
Derivative Assets (included in other assets on the consolidated balance sheets): |  |  |
Derivative Asset, Notional Amount | $ 941.2 |  | $ 571.0
Derivative Asset, Estimated Fair Value | 68.3 |  | 23.2
Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets): |  |  |
Derivative Liabilities, Notional Amount | 1,078.9 |  | 631.2
Derivative Liabilities, Estimated Fair Value | 63.1 |  | 22.2
Interest rate swap contracts |  |  |
Derivative Assets (included in other assets on the consolidated balance sheets): |  |  |
Derivative Asset, Notional Amount | 87.6 |  | 0.0
Derivative Asset, Estimated Fair Value | 0.2 |  | 0.0
Not Designated as Hedging Instrument | Interest rate swap contracts |  |  |
Derivative Assets (included in other assets on the consolidated balance sheets): |  |  |
Derivative Asset, Notional Amount | 754.3 |  | 503.2
Derivative Asset, Estimated Fair Value | 61.3 |  | 21.9
Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets): |  |  |
Derivative Liabilities, Notional Amount | 754.3 |  | 503.2
Derivative Liabilities, Estimated Fair Value | 61.3 |  | 21.9
Not Designated as Hedging Instrument | Interest rate lock commitments |  |  |
Derivative Assets (included in other assets on the consolidated balance sheets): |  |  |
Derivative Asset, Notional Amount | 186.9 |  | 67.8
Derivative Asset, Estimated Fair Value | 7.0 |  | 1.3
Not Designated as Hedging Instrument | Forward loan sales contracts |  |  |
Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets): |  |  |
Derivative Liabilities, Notional Amount | 237.0 |  | 128.0
Derivative Liabilities, Estimated Fair Value | $ 1.6 |  | $ 0.3
Interest rate swap contracts |  |  |
Derivative [Line Items] |  |  |
Derivative, Notional Amount, Contract 1 |  | $ 46.4 |
Derivative, Notional Amount, Contract 2 |  | 36.1 |
Derivative, Notional Amount |  | $ 5.1 |
Derivative, Fixed Interest Rate, Contract 1 |  | 0.0040 |
Derivatives, Fixed Interest Rate, Contract 2 |  | 0.0034 |
Derivatives, Fixed Interest Rate, Contract 3 |  | 0.0040 |

X
- Definition The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is YYYY-MM-DD. + References No definition available. + Details
Name: | dei_DocumentPeriodEndDate
Namespace Prefix: | dei_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Derivative, Fixed Interest Rate, Contract 1 + References No definition available. + Details
Name: | fibk_DerivativeFixedInterestRateContract1
Namespace Prefix: | fibk_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Derivative, Notional Amount, Contract 1 + References No definition available. + Details
Name: | fibk_DerivativeNotionalAmountContract1
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Derivative, Notional Amount, Contract 2 + References No definition available. + Details
Name: | fibk_DerivativeNotionalAmountContract2
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Derivative, Notional Amount, Contract 3 + References No definition available. + Details
Name: | fibk_DerivativeNotionalAmountContract3
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Derivatives, Fixed Interest Rate, Contract 2 + References No definition available. + Details
Name: | fibk_DerivativesFixedInterestRateContract2
Namespace Prefix: | fibk_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Derivatives, Fixed Interest Rate, Contract 3 + References No definition available. + Details
Name: | fibk_DerivativesFixedInterestRateContract3
Namespace Prefix: | fibk_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Nominal or face amount used to calculate payments on the derivative asset. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
Name: | us-gaap_DerivativeAssetNotionalAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DerivativeAssetsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_DerivativeLiabilitiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Nominal or face amount used to calculate payments on the derivative liability. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
Name: | us-gaap_DerivativeLiabilityNotionalAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
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_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Fair value as of the balance sheet date of all interest rate derivatives designated as cash flow hedging instruments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958 + Details
Name: | us-gaap_InterestRateCashFlowHedgeDerivativeAtFairValueNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value as of the balance sheet date of interest rate derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets. + References No definition available. + Details
Name: | us-gaap_InterestRateDerivativeAssetsAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value as of the balance sheet date of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as liabilities. + References No definition available. + Details
Name: | us-gaap_InterestRateDerivativeLiabilitiesAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of recognized loss from the hedging ineffectiveness of a fair value hedge. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=121577467&loc=d3e76258-113986 + Details
Name: | us-gaap_LossOnFairValueHedgeIneffectiveness
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditDerivativesByContractTypeAxis=us-gaap_InterestRateSwapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |