XML 137 R118.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Related Party Transactions (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Loans and Leases Receivable, Related Parties Disclosure [Abstract] |  |  |
Loan repayments | $ 18,300 |  |
Amount of loans removed due to changes in related parties | 1,100 |  |
Airplane Usage by Company |  |  |
Loans and Leases Receivable, Related Parties Disclosure [Abstract] |  |  |
Related party transaction, expenses from transactions with related party | 22 | $ 53 | $ 45
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 | 40,300 | 43,200 |
New loans and advances on existing loans | 16,500 |  |
Board of Directors Chairman | Airplane Usage by Company |  |  |
Loans and Leases Receivable, Related Parties Disclosure [Abstract] |  |  |
Related party transactions, other revenues | 30 | 25 | 17
Entity Majority Owned by Shareholders and Directors [Member] | Education, Communication, Strategic Enterprise Planning and Corporate Governance Consultation [Member] |  |  |
Loans and Leases Receivable, Related Parties Disclosure [Abstract] |  |  |
Related party transaction, expenses from transactions with related party | $ 85 | $ 80 | $ 73

X
- 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 -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://fasb.org/us-gaap/role/ref/legacyRef -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
Name: | us-gaap_LoansAndLeasesReceivableRelatedParties
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Reflects the amount of new advances made to related parties in the form of loan and lease receivables. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -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
Name: | us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_LoansAndLeasesReceivableRelatedPartiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of increase (decrease) in loans and leases receivable due from related parties. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -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_LoansAndLeasesReceivableRelatedPartiesPeriodIncreaseDecrease
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- 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/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(7)(e)(1)(i)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585 + Details
Name: | us-gaap_LoansAndLeasesReceivableRelatedPartiesProceeds
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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

X
- 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

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

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

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_BoardOfDirectorsChairmanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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