XML 75 R65.htm IDEA: XBRL DOCUMENT
v3.19.2

Regulatory Capital (Details) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Total risk-based capital: |  |
Actual, Amount | $ 1,420.5 | $ 1,285.0
Actual, Ratio | 13.43% | 12.99%
Minimum Required for Capital Adequacy Purposes, Amount | $ 846.0 | $ 791.2
Minimum Required for Capital Adequacy Purposes, Ratio | 8.00% | 8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 1,110.4 | $ 976.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 10.50% | 9.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 1,057.6 | $ 989.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 10.00% | 10.00%
Tier 1 risk-based capital: |  |
Actual, Amount | $ 1,346.3 | $ 1,212.0
Actual, Ratio | 12.73% | 12.26%
Minimum Required for Capital Adequacy Purposes, Amount | $ 634.5 | $ 593.4
Minimum Required for Capital Adequacy Purposes, Ratio | 6.00% | 6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 898.9 | $ 778.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 8.50% | 7.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 846.0 | $ 791.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 8.00% | 8.00%
Common equity tier 1 risk-based capital: |  |
Actual, Amount | $ 1,262.3 | $ 1,127.8
Actual, Ratio | 11.94% | 11.40%
Minimum Required for Capital Adequacy Purposes, Amount | $ 475.9 | $ 445.0
Minimum Required for Capital Adequacy Purposes, Ratio | 4.50% | 4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 740.3 | $ 630.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 7.00% | 6.375%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 687.4 | $ 642.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 6.50% | 6.50%
Leverage capital ratio: |  |
Actual, Amount | $ 1,346.3 | $ 1,212.0
Actual, Ratio | 9.94% | 9.47%
Capital Adequacy Purposes and Conservation Buffer, Amount | $ 541.7 | $ 511.9
Capital Adequacy Purposes and Conservation Buffer, Ratio | 4.00% | 4.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 677.2 | $ 639.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 5.00% | 5.00%
FIB |  |
Total risk-based capital: |  |
Actual, Amount | $ 1,294.6 | $ 1,184.5
Actual, Ratio | 12.27% | 12.01%
Minimum Required for Capital Adequacy Purposes, Amount | $ 844.3 | $ 788.8
Minimum Required for Capital Adequacy Purposes, Ratio | 8.00% | 8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 1,108.1 | $ 973.7
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 10.50% | 9.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 1,055.4 | $ 986.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 10.00% | 10.00%
Tier 1 risk-based capital: |  |
Actual, Amount | $ 1,220.4 | $ 1,111.6
Actual, Ratio | 11.56% | 11.27%
Minimum Required for Capital Adequacy Purposes, Amount | $ 633.2 | $ 591.6
Minimum Required for Capital Adequacy Purposes, Ratio | 6.00% | 6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 897.1 | $ 776.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 8.50% | 7.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 844.3 | $ 788.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 8.00% | 8.00%
Common equity tier 1 risk-based capital: |  |
Actual, Amount | $ 1,220.4 | $ 1,111.6
Actual, Ratio | 11.56% | 11.27%
Minimum Required for Capital Adequacy Purposes, Amount | $ 474.9 | $ 443.7
Minimum Required for Capital Adequacy Purposes, Ratio | 4.50% | 4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount | $ 738.8 | $ 628.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio | 7.00% | 6.375%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 686.0 | $ 640.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 6.50% | 6.50%
Leverage capital ratio: |  |
Actual, Amount | $ 1,220.4 | $ 1,111.6
Actual, Ratio | 9.01% | 8.97%
Capital Adequacy Purposes and Conservation Buffer, Amount | $ 541.9 | $ 495.9
Capital Adequacy Purposes and Conservation Buffer, Ratio | 4.00% | 4.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount | $ 677.4 | $ 619.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio | 5.00% | 5.00%

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 The amount of total capital as defined in the regulations. + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 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 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + 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 The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_CapitalRequiredForCapitalAdequacy
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
Name: | us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_CapitalRequiredToBeWellCapitalized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Total Capital divided by risk weighted assets as defined in the regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
Name: | us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + 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 The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Tier 1 Leverage Capital divided by average assets as defined in the regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
Name: | us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Tier 1 Risk Based Capital as defined in the regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + 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 The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Tier 1 capital divided by risk weighted assets as defined by regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
Name: | us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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