Fair Value Measurements Fair Value Inputs, Quantitative Information (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Loans and Leases Receivable, Impaired Loans \| Minimum \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(0.00%)	(0.00%)
Loans and Leases Receivable, Impaired Loans \| Maximum \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(51.00%)	(51.00%)
Loans and Leases Receivable, Impaired Loans \| Weighted Average \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(19.00%)	(19.00%)
Real Estate Acquired in Satisfaction of Debt \| Minimum \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(0.00%)	(0.00%)
Real Estate Acquired in Satisfaction of Debt \| Maximum \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(50.00%)	(50.00%)
Real Estate Acquired in Satisfaction of Debt \| Weighted Average \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(15.00%)	(15.00%)
Asses to be Disposed of by Sale \| Minimum \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(0.00%)	(0.00%)
Asses to be Disposed of by Sale \| Maximum \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(9.00%)	(9.00%)
Asses to be Disposed of by Sale \| Weighted Average \| Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	(5.00%)	(5.00%)
Estimated Fair Value \| Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	$ 44,940	$ 30,494
Long-lived assets to be disposed of by sale	2,083	1,083
Other real estate owned	$ 4,067	$ 4,554

Loans Receivable, Impaired, Fair Value Disclosure

No definition available.

Name:	fibk_LoansReceivableImpairedFairValueDisclosure
Namespace Prefix:	fibk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Real Estate Acquired Through Foreclosure, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	fibk_RealEstateAcquiredThroughForeclosureFairValueDisclosure
Namespace Prefix:	fibk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of assets classified as held for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19190-110258

+ Details

Name:	us-gaap_AssetsHeldForSaleLongLivedFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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_FairValueInputsAssetsQuantitativeInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueByAssetClassAxis=fibk_LoansandLeasesReceivableImpairedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_RealEstateAcquiredInSatisfactionOfDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: