XML 78 R69.htm IDEA: XBRL DOCUMENT
v3.23.2

Other Real Estate Owned (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Repossessed Assets [Abstract] |  |  |  |
Balance at beginning of year | $ 13.4 | $ 17.5 | $ 12.7 | $ 2.0
OREO acquired through acquisitions | 0.0 | 0.0 | 0.0 | 15.8
Additions | 1.7 | 0.1 | 2.5 | 0.1
Write-down of OREO | (0.7) | 0.0 | (0.8) | 0.0
Dispositions | 0.0 | (0.8) | 0.0 | (1.1)
Balance at end of year | $ 14.4 | $ 16.8 | $ 14.4 | $ 16.8

X
- Definition Real Estate Acquired Through Foreclosure, Acquired Through Acquisition + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Real Estate Acquired Through Foreclosure, Additions + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureAdditions
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Real Estate Acquired Through Foreclosure, Disposals + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureDisposals
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Real Estate Acquired Through Foreclosure, Valuation Adjustments + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureValuationAdjustments
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

X
- Definition Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11 -URI https://asc.fasb.org/extlink&oid=123577603&loc=d3e5162-111524 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=124259787&loc=d3e4437-111522 + Details
Name: | us-gaap_RealEstateAcquiredThroughForeclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant