XML 49 R39.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions |  |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Aug. 16, 2018 | May 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Class A common stock |  | $ 173.3 |  |  |  |  |  |
Goodwill |  |  |  | $ 546.4 | $ 445.0 | $ 546.4 | $ 444.7 | $ 212.8
Net increase to goodwill |  |  |  | $ 0.9 | $ 0.0 |  |  |
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents |  | 31.2 |  |  |  |  |  |
Investment securities |  | 3.1 |  |  |  |  |  |
Loans held for investment |  | 727.9 |  |  |  |  |  |
Allowance for loan loss |  | (8.0) |  |  |  |  |  |
Premises and equipment |  | 14.5 |  |  |  |  |  |
Other real estate owned (“OREO”) |  | 0.3 |  |  |  |  |  |
Core deposit intangible assets |  | 2.4 |  |  |  |  |  |
Deferred tax assets, net |  | 1.2 |  |  |  |  |  |
Other assets |  | 29.3 |  |  |  |  |  |
Total assets acquired |  | 801.9 |  |  |  |  |  |
Deposits |  | 696.1 |  |  |  |  |  |
Accounts Payable and Accrued Expense |  | 8.1 |  |  |  |  |  |
Long term debt |  | 13.0 |  |  |  |  |  |
Trust preferred securities |  | 5.2 |  |  |  |  |  |
Total liabilities assumed |  | 722.4 |  |  |  |  |  |
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 |  |  |  |  |  |
Loans held for investment | [1] | (14.8) |  |  |  |  |  |
Allowance for loan loss | [2] | 8.0 |  |  |  |  |  |
Premises and equipment | [3] | (0.5) |  |  |  |  |  |
Other real estate owned (“OREO”) |  | 0.3 |  |  |  |  |  |
Core deposit intangible assets | [4] | 13.3 |  |  |  |  |  |
Deferred tax assets, net | [5] | (1.6) |  |  |  |  |  |
Other assets | [6] | (9.7) |  |  |  |  |  |
Total assets acquired |  | (5.0) |  |  |  |  |  |
Deposits | [7] | 0.2 |  |  |  |  |  |
Accounts Payable and Accrued Expense | [8] | (0.4) |  |  |  |  |  |
Long term debt | [9] | 0.0 |  |  |  |  |  |
Trust preferred securities | [10] | (0.8) |  |  |  |  |  |
Total liabilities assumed |  | (1.0) |  |  |  |  |  |
Net assets acquired |  | (4.0) |  |  |  |  |  |
Northwest Bank Acquisition, as Recorded by the Company [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents |  | 31.2 |  |  |  |  |  |
Investment securities |  | 3.1 |  |  |  |  |  |
Loans held for investment |  | 713.1 |  |  |  |  |  |
Allowance for loan loss |  | 0.0 |  |  |  |  |  |
Premises and equipment |  | 14.0 |  |  |  |  |  |
Other real estate owned (“OREO”) |  | 0.6 |  |  |  |  |  |
Core deposit intangible assets |  | 15.7 |  |  |  |  |  |
Deferred tax assets, net |  | (0.4) |  |  |  |  |  |
Other assets |  | 19.6 |  |  |  |  |  |
Total assets acquired |  | 796.9 |  |  |  |  |  |
Deposits | [9] | 696.3 |  |  |  |  |  |
Accounts Payable and Accrued Expense | [9] | 7.7 |  |  |  |  |  |
Long term debt | [9] | 13.0 |  |  |  |  |  |
Trust preferred securities | [9] | 4.4 |  |  |  |  |  |
Total liabilities assumed |  | 721.4 |  |  |  |  |  |
Net assets acquired |  | 75.5 |  |  |  |  |  |
Cash |  | 3.0 |  |  |  |  |  |
Class A common stock |  | 173.3 |  |  |  |  |  |
Inland Northwest Bank |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total consideration paid |  | 176.3 |  |  |  |  |  |
Goodwill |  | $ 100.8 |  |  |  |  |  |
Cascade Bank Acquisition, As Recorded By Cascade Bank [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  | $ 246.8 |  |  |  |  |
Investment securities |  |  | 476.7 |  |  |  |  |
Loans held for investment |  |  | 2,111.0 |  |  |  |  |
Mortgage loans held for sale |  |  | 10.3 |  |  |  |  |
Allowance for loan loss |  |  | (24.0) |  |  |  |  |
Premises and equipment |  |  | 46.6 |  |  |  |  |
Other real estate owned (“OREO”) |  |  | 1.2 |  |  |  |  |
Core deposit intangible assets |  |  | 0.0 |  |  |  |  |
Deferred tax assets, net |  |  | 47.6 |  |  |  |  |
Other assets |  |  | 98.6 |  |  |  |  |
Total assets acquired |  |  | 3,014.8 |  |  |  |  |
Deposits |  |  | 2,669.9 |  |  |  |  |
Accounts Payable and Accrued Expense |  |  | 62.2 |  |  |  |  |
Total liabilities assumed |  |  | 2,732.1 |  |  |  |  |
Net assets acquired |  |  | 282.7 |  |  |  |  |
Cascade Bank Acquisition, Fair Value Adjustments [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  | 0.0 |  |  |  |  |
Investment securities | [11] |  | 4.9 |  |  |  |  |
Loans held for investment | [12] |  | (31.7) |  |  |  |  |
Mortgage loans held for sale |  |  | 0.0 |  |  |  |  |
Allowance for loan loss | [13] |  | 24.0 |  |  |  |  |
Premises and equipment | [14] |  | 0.1 |  |  |  |  |
Other real estate owned (“OREO”) |  |  | 0.0 |  |  |  |  |
Core deposit intangible assets | [15] |  | 48.0 |  |  |  |  |
Deferred tax assets, net | [16] |  | (20.9) |  |  |  |  |
Other assets |  |  | 2.1 |  |  |  |  |
Total assets acquired |  |  | 26.5 |  |  |  |  |
Deposits | [9] |  | (0.9) |  |  |  |  |
Accounts Payable and Accrued Expense | [17] |  | 1.9 |  |  |  |  |
Total liabilities assumed |  |  | 1.0 |  |  |  |  |
Net assets acquired |  |  | 25.5 |  |  |  |  |
Bank of the Cascades |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash |  |  | 155.0 |  |  |  |  |
Total consideration paid |  |  | 541.0 |  |  |  |  |
Goodwill |  |  | 232.8 |  |  |  |  |
Cascade Bank Acquisition, as Recorded by the Company [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  | 246.8 |  |  |  |  |
Investment securities |  |  | 481.6 |  |  |  |  |
Loans held for investment |  |  | 2,079.3 |  |  |  |  |
Mortgage loans held for sale |  |  | 10.3 |  |  |  |  |
Allowance for loan loss |  |  | 0.0 |  |  |  |  |
Premises and equipment |  |  | 46.7 |  |  |  |  |
Other real estate owned (“OREO”) |  |  | 1.2 |  |  |  |  |
Core deposit intangible assets |  |  | 48.0 |  |  |  |  |
Deferred tax assets, net |  |  | 26.7 |  |  |  |  |
Other assets |  |  | 100.7 |  |  |  |  |
Total assets acquired |  |  | 3,041.3 |  |  |  |  |
Deposits |  |  | 2,669.0 |  |  |  |  |
Accounts Payable and Accrued Expense |  |  | 64.1 |  |  |  |  |
Total liabilities assumed |  |  | 2,733.1 |  |  |  |  |
Net assets acquired |  |  | 308.2 |  |  |  |  |
Cash |  |  | 155.0 |  |  |  |  |
Class A common stock |  |  | 386.0 |  |  |  |  |
Total consideration paid |  |  | $ 541.0 |  |  |  |  |
Net increase to goodwill |  |  |  |  |  | $ 0.9 |  |
[1] | (1) Write down of the book value of loans to their estimated fair values. The fair value of the loans was estimated using cash flow projections based on the remaining maturity and repricing terms, adjusted for estimated future credit losses and prepayments and discounted to present value using a risk-adjusted market rate for similar loans. The fair value of collateral dependent loans acquired with deteriorated credit quality was estimated based on the Company’s analysis of the fair value of each loan’s underlying collateral, discounted using market-derived rates of return with consideration given to the period of time and costs associated with foreclosure and disposition of the collateral.
[2] | (2) Adjustment to remove the Northwest allowance for loan losses at acquisition date, as the credit risk is included in the fair value adjustment for loans receivable described in (1) above.
[3] | (3) Write down of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon broker’s opinion of value.
[4] | (4) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm.
[5] | (5) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition.
[6] | (6) Adjustment consists of reductions to the fair value of other items, including the removal of Northwest previously recorded goodwill.
[7] | (7) Increase in book value of time deposits to their estimated fair values based upon interest rates of similar time deposits with similar terms on the date of acquisition based upon valuation from an independent accounting and advisory firm.
[8] | (8) Decrease due to the write-off of off balance sheet reserves.
[9] | Decrease in book value of time deposits to their estimated fair values based upon interest rates of similar time deposits with similar terms on the date of acquisition based upon valuation from an independent accounting and advisory firm.
[10] | (9) Write down of the book value of debt to the estimated fair values on the date of acquisition based upon favorable interest rates in the market.
[11] | Write up of the book value of investments to their estimated fair values on the date of acquisition based upon quotes obtained from an independent third party pricing service.
[12] | Write down of the book value of loans to their estimated fair values. Shared National Credits (SNC) were recorded at quoted sales prices where available. The fair value of the remaining loans was estimated using cash flow projections based on the remaining maturity and repricing terms, adjusted for estimated future credit losses and prepayments and discounted to present value using a risk-adjusted market rate for similar loans. The fair value of collateral dependent loans acquired with deteriorated credit quality was estimated based on the Company’s analysis of the fair value of each loan’s underlying collateral, discounted using market-derived rates of return with consideration given to the period of time and costs associated with foreclosure and disposition of the collateral.
[13] | Adjustment to remove the Cascade allowance for loan losses at acquisition date, as the credit risk is accounted for in the fair value adjustment for loans receivable described in (2) above.
[14] | Write up of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon appraisals obtained from an independent third party appraiser or broker’s opinion of value.
[15] | Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm.
[16] | Adjustment consists of the write-off of pre-existing deferred tax assets as a result of the acquisition.
[17] | Increase in fair value due to credit card incentive program, unfavorable leases, write-off of balance sheet reserve, and swap liability offset.

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, 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, 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, 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 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 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 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 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 Investment + References No definition available. + Details
Name: | fibk_BusinessCombinationsRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedLoansHeldforInvestment
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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 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 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=109124213&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=109124213&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=84161108&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=84161108&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=84161108&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=108330185&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=84161108&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=84161108&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=108330185&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=84161108&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=84161108&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=108330185&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=84161108&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 Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the 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 805 -SubTopic 10 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 + Details
Name: | us-gaap_GoodwillPurchaseAccountingAdjustments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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=109223946&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_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_NorthwestBankAcquisitionasRecordedbytheCompanyMember
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_BusinessAcquisitionAxis=fibk_CascadeBankAcquisitionAsRecordedByCascadeBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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