XML 86 R61.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) $ / shares in Units, $ in Millions |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Apr. 08, 2019 USD ($) segment office $ / shares shares | Aug. 16, 2018 USD ($) office $ / shares shares | Dec. 31, 2019 USD ($) segment | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Business Acquisition [Line Items] |  |  |  |  |
Goodwill |  |  | $ 621.6 | $ 546.7 | $ 444.7
Number of operating segments | segment |  |  | 1 |  |
Idaho Independent Bank, Fair Value Adjustments [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Cash and cash equivalents | $ 0.0 |  |  |  |
Investment securities | 0.5 |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment | (9.8) |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale | 0.0 |  |  |  |
Allowance for loan losses | 6.3 |  |  |  |
Premises and equipment | 4.8 |  |  |  |
Other real estate owned (“OREO”) | 2.0 |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned | 0.0 |  |  |  |
Core deposit intangible assets | 13.6 |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset | (2.6) |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense | 2.6 |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets | (0.7) |  |  |  |
Total assets acquired | 14.1 |  |  |  |
Deposits | 0.1 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities | 0.1 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements | 0.0 |  |  |  |
Total liabilities assumed | 2.8 |  |  |  |
Net assets acquired | $ 11.3 |  |  |  |
Idaho Independent Bank [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of banking offices | office | 11 |  |  |  |
Cash and cash equivalents | $ 270.7 |  |  |  |
Investment securities | 63.2 |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment | 337.8 |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale | 0.5 |  |  |  |
Allowance for loan losses | 0.0 |  |  |  |
Premises and equipment | 21.3 |  |  |  |
Other real estate owned (“OREO”) | 2.4 |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned | 15.2 |  |  |  |
Core deposit intangible assets | 13.6 |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset | 0.6 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense | 17.8 |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets | 7.9 |  |  |  |
Total assets acquired | 733.2 |  |  |  |
Deposits | 596.6 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities | 4.1 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements | 30.4 |  |  |  |
Total liabilities assumed | 648.9 |  |  |  |
Net assets acquired | 84.3 |  |  |  |
Class A common stock | 157.3 |  |  |  |
Total consideration paid | 157.3 |  |  |  |
Goodwill | $ 73.0 |  |  |  |
Idaho Independent Bank [Member] | Class A Common Stock |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business Acquisition Conversion Share Rate | shares | 0.50 |  |  |  |
Common stock issued (in shares) | shares | 3,871,422 |  |  |  |
Business Acquisition, Share Price | $ / shares | $ 40.64 |  |  |  |
Idaho Independent Bank, As Recorded By Idaho Independent Bank [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale | $ 0.5 |  |  |  |
Allowance for loan losses | (6.3) |  |  |  |
Premises and equipment | 16.5 |  |  |  |
Other real estate owned (“OREO”) | 0.4 |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned | 15.2 |  |  |  |
Core deposit intangible assets | 0.0 |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset | 3.2 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense | 15.2 |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets | 8.6 |  |  |  |
Total assets acquired | 719.1 |  |  |  |
Deposits | 596.5 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities | 4.0 |  |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements | 30.4 |  |  |  |
Total liabilities assumed | 646.1 |  |  |  |
Net assets acquired | $ 73.0 |  |  |  |
Community 1st Bank [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of banking offices | office | 3 |  |  |  |
Total consideration paid | $ 18.8 |  |  |  |
Goodwill | $ 2.3 |  |  |  |
Community 1st Bank [Member] | Class A Common Stock |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business Acquisition Conversion Share Rate | shares | 0.3784 |  |  |  |
Common stock issued (in shares) | shares | 463,134 |  |  |  |
Business Acquisition, Share Price | $ / shares | $ 40.64 |  |  |  |
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Cash and cash equivalents | $ 270.7 | $ 31.2 |  |  |
Investment securities | 62.7 | 3.1 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment | $ 347.6 | 727.9 |  |  |
Allowance for loan losses |  | (8.0) |  |  |
Premises and equipment |  | 14.5 |  |  |
Other real estate owned (“OREO”) |  | 0.3 |  |  |
Core deposit intangible assets |  | 2.4 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets |  | 29.3 |  |  |
Total assets acquired |  | 800.7 |  |  |
Deposits |  | 696.1 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities |  | 8.1 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt |  | 13.0 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities |  | 5.2 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability |  | (1.2) |  |  |
Total liabilities assumed |  | 721.2 |  |  |
Net assets acquired |  | 79.5 |  |  |
Northwest Bank Acquisition, Fair Value Adjustments [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Cash and cash equivalents |  | 0.0 |  |  |
Investment securities |  | 0.0 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment |  | (14.8) |  |  |
Allowance for loan losses |  | 8.0 |  |  |
Premises and equipment |  | 0.0 |  |  |
Other real estate owned (“OREO”) |  | 0.3 |  |  |
Core deposit intangible assets |  | 13.3 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets |  | (10.0) |  |  |
Total assets acquired |  | (3.2) |  |  |
Deposits |  | 0.2 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities |  | (0.4) |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt |  | 0.1 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities |  | (0.8) |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability |  | 1.6 |  |  |
Total liabilities assumed |  | 0.7 |  |  |
Net assets acquired |  | $ (3.9) |  |  |
Inland Northwest Bank [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of banking offices | office |  | 20 |  |  |
Cash and cash equivalents |  | $ 31.2 |  |  |
Investment securities |  | 3.1 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment |  | 713.1 |  |  |
Allowance for loan losses |  | 0.0 |  |  |
Premises and equipment |  | 14.5 |  |  |
Other real estate owned (“OREO”) |  | 0.6 |  |  |
Core deposit intangible assets |  | 15.7 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets |  | 19.3 |  |  |
Total assets acquired |  | 797.5 |  |  |
Deposits |  | 696.3 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities |  | 7.7 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt |  | 13.1 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities |  | 4.4 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability |  | 0.4 |  |  |
Total liabilities assumed |  | 721.9 |  |  |
Net assets acquired |  | 75.6 |  |  |
Cash |  | 3.0 |  |  |
Class A common stock |  | 173.3 |  |  |
Total consideration paid |  | 176.3 |  |  |
Goodwill |  | $ 100.7 |  |  |
Number of operating segments | segment | 1 |  |  |  |
Inland Northwest Bank [Member] | Class A Common Stock |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business Acquisition Conversion Share Rate | shares |  | 0.516 |  |  |
Common stock issued (in shares) | shares |  | 3,840,000 |  |  |
Business Acquisition, Share Price | $ / shares |  | $ 45.15 |  |  |
Core Deposits Intangibles |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |  |
Core Deposits Intangibles | Idaho Independent Bank [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |  |
Core Deposits Intangibles | Inland Northwest Bank [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of related deposits |  | 10 years |  |  |

X
- Definition Business Acquisition Conversion Share Rate + References No definition available. + Details
Name: | fibk_BusinessAcquisitionConversionShareRate
Namespace Prefix: | fibk_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAccountsPayableAndAccruedExpense
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Deposits + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeposits
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLifeInsuranceCorporateOrBankOwned
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans and Leases Receivable, Allowance + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoansAndLeasesReceivableAllowance
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssets
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherLiabilities
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedSecuritiesSoldUnderRepurchaseAgreements
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentLiabilitiesDeferredTaxLiability
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities + References No definition available. + Details
Name: | fibk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentLiabilitiesTrustPreferredSecurities
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combinations, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Asset + References No definition available. + Details
Name: | fibk_BusinessCombinationsRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAsset
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale + References No definition available. + Details
Name: | fibk_BusinessCombinationsRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoansHeldForSale
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Combinations, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans Held for Investment + References No definition available. + Details
Name: | fibk_BusinessCombinationsRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoansHeldforInvestment
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Combinations Recognized Identifiable Assets Acquired And Liabilities Assumed Other Real Estate Owned + References No definition available. + Details
Name: | fibk_BusinessCombinationsRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherRealEstateOwned
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Number Of Banking Offices Acquired + References No definition available. + Details
Name: | fibk_NumberOfBankingOfficesAcquired
Namespace Prefix: | fibk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
Name: | us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
Name: | us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Number of shares of equity interests issued or issuable to acquire entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
Name: | us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionSharePrice
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477 + Details
Name: | us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of assets acquired at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of identifiable intangible assets recognized as of the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=116868840&loc=d3e2207-128464 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of liabilities assumed at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=116868840&loc=d3e2207-128464 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of long-term debt due after one year or the normal operating cycle, if longer, assumed at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of property, plant, and equipment recognized as of the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=116868840&loc=d3e2207-128464 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
Name: | us-gaap_Goodwill
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585 + Details
Name: | us-gaap_PaymentsToAcquireBusinessesGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_IdahoIndependentBankFairValueAdjustmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_IdahoIndependentBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_IdahoIndependentBankAsRecordedByIdahoIndependentBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_Community1stBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_NorthwestBankAcquisitionAsRecordedByInlandNorthwestBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_NorthwestBankAcquisitionFairValueAdjustmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fibk_InlandNorthwestBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CoreDepositsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |