Shares Acquired

No definition available.

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Namespace Prefix:	fibk_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
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+ Details

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

- Definition

Reflects the amount of new advances made to related parties in the form of loan and lease receivables.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase (decrease) in loans and leases receivable due from related parties.

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableRelatedPartiesPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash inflow from related parties which reduce the amount of loans and leases receivable due from related parties.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 12
-Subparagraph (a)
-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
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-SubTopic 210
-Topic 942
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow to reacquire common stock during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 15
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-15

+ Details

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

- Definition

Amount of transactions with related party during the financial reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-3

+ Details

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

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
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-Publisher FASB
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-Topic 946
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-Name Accounting Standards Codification
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-Topic 946
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-Name Accounting Standards Codification
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-Paragraph 1
-Subparagraph (SX 210.6-07(2)(g)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

+ Details

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

- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481549/505-30-45-1

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fibk_ExecutiveOfficersDirectorsShareholdersGreaterThanFivePercentAndRelatedEntitiesAndIndividualsToSuchPersonsMember
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- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
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Related Party Transactions (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Dec. 14, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Related Party Transaction
Shares Acquired, Average Cost Per Share	$ 32.14
Payments for Repurchase of Common Stock	$ 32.1	$ 1.2	$ 34.0	$ 199.0
Amount of loans removed due to changes in related parties		0.2
Related Party Transaction, Amounts of Transaction		0.4
Executive Officers, Directors, Shareholders Greater than Five Percent, and Related Entities and Individuals to Such Persons [Member]
Related Party Transaction
Loans and leases receivable, related parties		5.1	$ 11.1
New loans and advances on existing loans		3.0
Loan repayments		$ 8.8
Class A Common Stock
Related Party Transaction
Shares Acquired	1,000,000