Ownership Percentage Threshhold for Related Party

No definition available.

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- Definition

Related Party Relationship, Number of Directors

+ References

No definition available.

+ Details

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

- Definition

Related Party Transaction, Contractual Agreement, Amount

+ References

No definition available.

+ Details

Name:	fibk_RelatedPartyTransactionContractualAgreementAmount
Namespace Prefix:	fibk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Related Party Transaction, Contractual Agreement, Initial Payment

+ References

No definition available.

+ Details

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Namespace Prefix:	fibk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Related Party Transaction, Contractual Agreement, Percentage of Contract Price

+ References

No definition available.

+ Details

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

- Definition

Stock Repurchased and Retired During Period, Weighted Average Price

+ References

No definition available.

+ Details

Name:	fibk_StockRepurchasedandRetiredDuringPeriodWeightedAveragePrice
Namespace Prefix:	fibk_
Data Type:	num:perShareItemType
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

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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

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

- Definition

Reflects the amount of collections received from related parties to reduce the carrying amounts of loans and leases receivable due from them.

+ 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 (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: 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

+ Details

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

Reflects the sum for the period of the additions and reductions to related party loans, which when added to the opening balance of related party loan will agree to the ending balance of such loans. This subtotal typically is not reflected.

+ References

+ Details

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

- Definition

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

+ References

+ Details

Name:	us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
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

No definition available.

+ Details

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

- Definition

Amount of selling, general and administrative expenses resulting from transactions, excluding transactions that are eliminated in consolidated or combined financial statements, with related party.

+ References

No definition available.

+ Details

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

- Definition

Number of shares that have been repurchased and retired during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Article 3

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 29, 30
-Article 5

+ Details

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

- 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
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- Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fibk_InsurancePremiumsMember
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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=fibk_EntityMajorityOwnedByShareholdersAndDirectorsMember
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Data Type:	na
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- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fibk_EducationCommunicationStrategicEnterprisePlanningAndCorporateGovernanceConsultationMember
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- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fibk_DirectorWhollyOwnedEntityMember
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Data Type:	na
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Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fibk_ConstructionManagementAndAdivsoryServicesMember
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Period Type:

- Details

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- Details

Name:	us-gaap_RelatedPartyTransactionAxis=us-gaap_DirectorMember
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Related Party Transactions (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) director $ / shares shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Shareholder
Related Party Transaction
Ownership percentage threshhold for related party	5.00%
Executive Officers, Directors, Shareholders Greater than Five Percent, and Related Entities and Individuals to Such Persons [Member]
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loans and leases receivable, related parties	$ 33,008	$ 33,002
Loans and leases receivable, related parties, additions	7,400
Loans and leases receivable, related parties, collections	7,283
Loans and Leases Receivable, Related Parties, Period Increase (Decrease)	111
Director, Controlling Ownership Interest in Entity \| Insurance Premiums
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, selling, general and administrative expenses from transactions with related party			$ 764
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	332	306	309
Related party transactions, other revenues	$ 64	77	61
Entity Majority Owned by Shareholders and Directors
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party relationship, number of directors \| director	6
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	$ 210	255	224
Director, Wholly-Owned Entity \| Construction Management and Adivsory Services
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party		$ 60	$ 60
Related party transaction, contractual agreement, amount	$ 180
Related party transaction, contractual agreement, percentage of contract price	4.00%
Related party transaction, contractual agreement, initial payment	$ 60
Class A Common Stock
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Stock Repurchased and Retired During Period, Shares \| shares	765,875
Stock Repurchased and Retired During Period, Weighted Average Price \| $ / shares	$ 26.01
Class A Common Stock \| Director
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Stock Repurchased and Retired During Period, Shares \| shares	37,615
Stock Repurchased and Retired During Period, Weighted Average Price \| $ / shares	$ 24.96