XML 140 R90.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Mortgage Servicing Rights - Narrative (Details) - Mortgage loans - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2016 | Dec. 31, 2015
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract] |  |  |  |
Mortgage servicing rights, fair value | $ 34,800,000 | $ 42,400,000 |  |
Servicing asset at fair value, assumptions used to estimate fair value, weighted average life | 6 years | 8 years |  |
Impairment reversals | $ 0 |  |  |
Permanent impairment charged against carrying value | $ 0 |  | $ 0 | $ 0
Minimum |  |  |  |
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract] |  |  |  |
Servicing assets at fair value, assumptions used to estimate fair value, discount rate | 9.30% | 10.40% |  |
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed | 0.60% | 0.40% |  |
Maximum [Member] |  |  |  |
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract] |  |  |  |
Servicing assets at fair value, assumptions used to estimate fair value, discount rate | 11.00% | 12.10% |  |
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed | 1.50% | 1.50% |  |

X
- Definition Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122625-111746 + Details
Name: | us-gaap_ServicingAssetAtFairValueAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

X
- Definition Discount rate used to estimate the fair value of servicing assets and servicing liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122596-111746 + Details
Name: | us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Estimated rate of prepayments of principal on servicing assets and servicing liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122596-111746 + Details
Name: | us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average life of financial assets and financial liabilities, 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 860 -SubTopic 50 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122596-111746 + Details
Name: | us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueWeightedAverageLife1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of decrease in the valuation allowance for impairment of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer from write-downs charged against the allowance. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (e)(3) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746 + Details
Name: | us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsAggregateWriteDowns
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of decrease in the valuation allowance for impairment of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer from recoveries recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (e)(2) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746 + Details
Name: | us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fibk_MortgageLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |