Ownership Percentage Threshold for Related Party

No definition available.

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Namespace Prefix:	fibk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Related Party Relationship, Number of Directors

+ References

No definition available.

+ Details

Name:	fibk_RelatedPartyRelationshipNumberOfDirectors
Namespace Prefix:	fibk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Related Party Transaction, Aggregate Ownership Interest

+ References

No definition available.

+ Details

Name:	fibk_RelatedPartyTransactionAggregateOwnershipInterest
Namespace Prefix:	fibk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Related Party Transaction, Percentage Of Aircraft Owned

+ References

No definition available.

+ Details

Name:	fibk_RelatedPartyTransactionPercentageOfAircraftOwned
Namespace Prefix:	fibk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(e))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

Name:	us-gaap_LoansAndLeasesReceivableRelatedParties
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

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

- References

No definition available.

+ Details

Name:	us-gaap_LoansAndLeasesReceivableRelatedPartiesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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

+ Details

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

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Definition

Reflects the sum of all other revenue and income realized from sales and other transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fibk_EducationCommunicationStrategicEnterprisePlanningAndCorporateGovernanceConsultationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fibk_ShareholderMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fibk_ExecutiveOfficersDirectorsShareholdersGreaterThanFivePercentAndRelatedEntitiesAndIndividualsToSuchPersonsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fibk_BoardOfDirectorsChairmanWhollyOwnedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fibk_AircraftUsageAndRelatedActivitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_BoardOfDirectorsChairmanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fibk_AirplaneusagebyCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fibk_Onedirectorandonegreaterthan10shareholderMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=us-gaap_PaymentGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fibk_EntityMajorityOwnedByShareholdersAndDirectorsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions (Details)	12 Months Ended
Related Party Transactions (Details)	Dec. 31, 2017 USD ($) director	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Percentage of aircraft owned	66.00%
Number of directors \| director	5
Education, Communication, Strategic Enterprise Planning and Corporate Governance Consultation
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Aggregate ownership interest of chairman and board of directors	15.00%
Shareholder
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Ownership percentage threshold for related party	5.00%
Executive Officers, Directors, Shareholders Greater than Five Percent, and Related Entities and Individuals to Such Persons
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loans and leases receivable, related parties	$ 54,600,000	$ 53,300,000
New loans and advances on existing loans	15,000,000
Loan repayments	15,700,000
Amount of loans removed due to changes in related parties	2,000,000
Board of Directors Chairman, Wholly-Owned Entity \| Aircraft Usage and Related Activities
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	0	108,000	$ 332,000
Related party transactions, other revenues	0	53,000	64,000
Board of Directors Chairman \| Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	45,000	121,000	28,000
Related party transactions, other revenues	17,000	19,000	23,000
One director and one greater than 10% shareholder \| Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	17,000	175,000
One director and one greater than 10% shareholder \| Payment Guarantee
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loans and leases receivable, related parties	1,900,000	2,000,000
Entity Majority Owned by Shareholders and Directors \| Education, Communication, Strategic Enterprise Planning and Corporate Governance Consultation
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	$ 73,000	$ 100,000	$ 210,000