Amount of allowance for credit loss on accounts receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5074-111524

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

Name:	us-gaap_AllowanceForDoubtfulAccountsReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reflects the aggregate value of loans, trade receivables, securities and financial instruments that serve as collateral for obligations arising from a transfer of financial assets recorded as a secured borrowing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5086-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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

- Definition

Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

+ References

+ Details

Name:	us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

Name:	us-gaap_ProvisionForLoanAndLeaseLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_AgriculturePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_RealEstateFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_AgriculturalFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=fibk_BusinessAssetsPledgedasCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=fibk_RealPropertyPledgedasCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=srt_OtherPropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans Held for Investment - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	$ 0.0	$ (19.5)	$ 5.1	$ (48.5)
Accounts Receivable, Allowance for Credit Loss	135.5	146.1	135.5	146.1	$ 136.6	$ 144.3	$ 129.1	$ 73.0
Collateral dependent borrowings	15.7		15.7			17.5
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	0.0	19.3		48.5
Balance of defaulted loans under trouble debt restructurings	0.0		0.0
Impaired Financing Receivable, Interest Income, Accrual Method	0.0	0.0	0.0	0.0
Agriculture Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	0.0	(0.1)		0.0
Accounts Receivable, Allowance for Credit Loss	0.4	0.9	0.4	0.9	0.6		1.1
Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(3.7)	(10.0)	(0.8)	(18.1)
Accounts Receivable, Allowance for Credit Loss	79.4	75.2	79.4	75.2	77.7	80.5	65.0	28.9
Collateral dependent borrowings	10.9		10.9			8.9
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(3.8)	(8.6)	(5.6)	(24.9)
Accounts Receivable, Allowance for Credit Loss	33.7	46.5	33.7	46.5	36.0	39.2	38.2	32.6
Collateral dependent borrowings	4.1		4.1			7.8
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	0.0	(0.1)	(0.1)	0.0
Accounts Receivable, Allowance for Credit Loss	0.4	$ 0.9	0.4	$ 0.9	$ 0.6	0.7	$ 1.1	$ 1.6
Collateral dependent borrowings	0.7		0.7			0.8
Business Assets Pledged as Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	4.4		4.4			7.4
Business Assets Pledged as Collateral [Member] \| Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	1.3		1.3			1.3
Business Assets Pledged as Collateral [Member] \| Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	3.1		3.1			6.1
Business Assets Pledged as Collateral [Member] \| Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			0.0
Real Property Pledged as Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	11.3		11.3			8.6
Real Property Pledged as Collateral [Member] \| Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	9.6		9.6			6.5
Real Property Pledged as Collateral [Member] \| Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	1.0		1.0			1.3
Real Property Pledged as Collateral [Member] \| Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.7		0.7			0.8
Other Property [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			1.5
Other Property [Member] \| Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			1.1
Other Property [Member] \| Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			0.4
Other Property [Member] \| Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	$ 0.0		$ 0.0			$ 0.0