XML 70 R60.htm IDEA: XBRL DOCUMENT
v3.20.2

Regulatory Capital (Details) $ in Millions | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Jan. 01, 2020 USD ($) | Dec. 31, 2019 USD ($)
--- | --- | --- | --- | ---
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |
Retained Earnings (Accumulated Deficit) | $ 912.5 |  |  | $ 953.6
Total risk-based capital: |  |  |  |
Actual, Amount | $ 1,600.1 |  |  | $ 1,495.3
Actual, Ratio | 0.1476 |  |  | 0.1410
Minimum Required for Capital Adequacy Purposes, Amount | $ 867.6 |  |  | $ 848.5
Minimum Required for Capital Adequacy Purposes, Ratio | 0.08000 |  |  | 0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 1,138.7 |  |  | $ 1,113.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 10.50% |  |  | 10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 1,084.4 |  |  | $ 1,060.6
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 0.1000 |  |  | 0.1000
Tier 1 risk-based capital: |  |  |  |
Actual, Amount | $ 1,393.1 |  |  | $ 1,422.3
Actual, Ratio | 0.1285 |  |  | 0.1341
Minimum Required for Capital Adequacy Purposes, Amount | $ 650.7 |  |  | $ 636.3
Minimum Required for Capital Adequacy Purposes, Ratio | 0.06000 |  |  | 0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 921.8 |  |  | $ 901.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 8.50% |  |  | 8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 867.6 |  |  | $ 848.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 0.0800 |  |  | 0.0800
Common equity tier 1 risk-based capital: |  |  |  |
Actual, Amount | $ 1,309.0 |  |  | $ 1,338.2
Actual, Ratio | 12.07% |  |  | 12.62%
Minimum Required for Capital Adequacy Purposes, Amount | $ 488.0 |  |  | $ 477.3
Minimum Required for Capital Adequacy Purposes, Ratio | 4.50% |  |  | 4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 759.1 |  |  | $ 742.4
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 7.00% |  |  | 7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 704.9 |  |  | $ 689.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 6.50% |  |  | 6.50%
Leverage capital ratio: |  |  |  |
Actual, Amount | $ 1,393.1 |  |  | $ 1,422.3
Actual, Ratio | 0.0922 |  |  | 0.1013
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum | $ 604.6 |  |  | $ 561.6
Capital Adequacy Purposes and Conservation Buffer, Ratio | 0.04000 |  |  | 0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 755.8 |  |  | $ 702.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 0.0500 |  |  | 0.0500
FIB |  |  |  |
Total risk-based capital: |  |  |  |
Actual, Amount | $ 1,404.5 |  |  | $ 1,321.4
Actual, Ratio | 0.1299 |  |  | 0.1250
Minimum Required for Capital Adequacy Purposes, Amount | $ 865.0 |  |  | $ 845.8
Minimum Required for Capital Adequacy Purposes, Ratio | 0.08000 |  |  | 0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 1,135.3 |  |  | $ 1,110.1
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 10.50% |  |  | 10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 1,081.2 |  |  | $ 1,057.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 0.1000 |  |  | 0.1000
Tier 1 risk-based capital: |  |  |  |
Actual, Amount | $ 1,297.4 |  |  | $ 1,248.4
Actual, Ratio | 0.1200 |  |  | 0.1181
Minimum Required for Capital Adequacy Purposes, Amount | $ 648.7 |  |  | $ 634.3
Minimum Required for Capital Adequacy Purposes, Ratio | 0.06000 |  |  | 0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 919.0 |  |  | $ 898.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 8.50% |  |  | 8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 865.0 |  |  | $ 845.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 0.0800 |  |  | 0.0800
Common equity tier 1 risk-based capital: |  |  |  |
Actual, Amount | $ 1,297.4 |  |  | $ 1,248.4
Actual, Ratio | 12.00% |  |  | 11.81%
Minimum Required for Capital Adequacy Purposes, Amount | $ 486.5 |  |  | $ 475.7
Minimum Required for Capital Adequacy Purposes, Ratio | 4.50% |  |  | 4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 756.8 |  |  | $ 740.0
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 7.00% |  |  | 7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 702.8 |  |  | $ 687.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 6.50% |  |  | 6.50%
Leverage capital ratio: |  |  |  |
Actual, Amount | $ 1,297.4 |  |  | $ 1,248.4
Actual, Ratio | 0.0860 |  |  | 0.0891
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum | $ 603.4 |  |  | $ 560.4
Capital Adequacy Purposes and Conservation Buffer, Ratio | 0.04000 |  |  | 0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 754.3 |  |  | $ 700.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 0.0500 |  |  | 0.0500
Accounting Standards Update 2016-13 [Member] |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |
Retained Earnings (Accumulated Deficit) |  | $ (24.1) |  |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |
Retained Earnings (Accumulated Deficit) |  |  | $ (24.1) |

X
- Definition Capital Required for Capital Adequacy under Basel III Fully Phased-in + References No definition available. + Details
Name: | fibk_CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets + References No definition available. + Details
Name: | fibk_CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule + References No definition available. + Details
Name: | fibk_CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInSchedule
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets + References No definition available. + Details
Name: | fibk_CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInScheduleToRiskWeightedAssets
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Common Equity Tier One Capital to be Well Capitalized + References No definition available. + Details
Name: | fibk_CommonEquityTierOneCapitalToBeWellCapitalized
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets + References No definition available. + Details
Name: | fibk_CommonEquityTierOneCapitalToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Common Equity Tier One Risk Based Capital to Risk Weighted Assets + References No definition available. + Details
Name: | fibk_CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Common Equity Tier One Risked Based Capital + References No definition available. + Details
Name: | fibk_CommonEquityTierOneRiskedBasedCapital
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Common equity tier one risk-based capital [Abstract] + References No definition available. + Details
Name: | fibk_CommonequitytieroneriskbasedcapitalAbstract
Namespace Prefix: | fibk_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in + References No definition available. + Details
Name: | fibk_TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets + References No definition available. + Details
Name: | fibk_TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in + References No definition available. + Details
Name: | fibk_TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets + References No definition available. + Details
Name: | fibk_TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of total capital as defined by regulatory framework. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998 + Details
Name: | us-gaap_Capital
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Definition Amount of minimum total risk-based capital required for capital adequacy as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826 + Details
Name: | us-gaap_CapitalRequiredForCapitalAdequacy
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Ratio of minimum total risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of minimum total risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_CapitalRequiredToBeWellCapitalized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
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_ComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cumulative amount of the reporting entity's undistributed earnings or deficit. + 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(30)(a)(3)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 65 -Paragraph 15 -Subparagraph (g)(2) -URI http://asc.fasb.org/extlink&oid=121322162&loc=SL121327923-165333 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(23)(a)(4)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=121370832&loc=SL117420844-207641 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (g)(2)(i) -URI http://asc.fasb.org/extlink&oid=121370832&loc=SL117420844-207641 + Details
Name: | us-gaap_RetainedEarningsAccumulatedDeficit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of Tier 1 leverage capital as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapital
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Definition Amount of minimum Tier 1 leverage capital required for capital adequacy as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Ratio of minimum Tier 1 leverage capital to average assets required for capital adequacy as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of minimum Tier 1 leverage capital categorized as well capitalized as defined by regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Ratio of minimum Tier 1 leverage capital to average assets categorized as well capitalized as defined by regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio of Tier 1 capital to average assets as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of Tier 1 risk-based capital as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapital
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Definition Amount of minimum Tier 1 risk-based capital required for capital adequacy as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Ratio of minimum Tier 1 risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of minimum Tier 1 risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Ratio of minimum Tier 1 risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio of Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | dei_LegalEntityAxis=fibk_FirstInterstateBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CumulativeEffectPeriodOfAdoptionAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |