XML 92 R84.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Other Real Estate Owned (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Repossessed Assets [Abstract] |  |  |
Real Estate Acquired Through Foreclosure, Valuation Adjustments | $ 0.9 | $ 0.1 |
Real Estate Acquired Through Foreclosure [Roll Forward] |  |  |
Balance at beginning of year | 14.4 | 10.1 | $ 10.0
Acquisitions | 2.4 | 0.6 | 1.2
Additions | 14.1 | 12.1 | 5.4
Valuation adjustments | (0.9) | (0.1) | (0.4)
Dispositions | (21.8) | (8.3) | (6.1)
Balance at end of year | 8.5 | 14.4 | 10.1
Real Estate Acquired Through Foreclosure, Improvements | 0.3 |  | 0.0
Write-downs of OREO | $ (0.9) | $ (0.1) | $ (0.4)

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, Improvements + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureImprovements
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Real Estate Acquired Through Foreclosure [Roll Forward] + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureRollForward
Namespace Prefix: | fibk_
Data Type: | xbrli:stringItemType
Balance Type: | na
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 The charge against earnings in the period to reduce the carrying amount of real property to fair value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2921-110230 + Details
Name: | us-gaap_ImpairmentOfRealEstate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 http://asc.fasb.org/extlink&oid=118936363&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 http://asc.fasb.org/extlink&oid=118952595&loc=d3e4437-111522 + Details
Name: | us-gaap_RealEstateAcquiredThroughForeclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant