XML 30 R49.htm IDEA: XBRL DOCUMENT
v3.19.3

Investment Securities - Narrative (Details) |  | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Jan. 01, 2019 USD ($) | Sep. 30, 2019 USD ($) Investments | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) Investments | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |  |  |
Investment securities classified as held to maturity |  | $ 94,900,000 |  | $ 94,900,000 |  | $ 406,800,000
Amortized cost of held to maturity securities transferred | $ 281,100,000 |  |  |  |  |
Fair value of held to maturity securities | $ 275,000,000.0 |  |  |  |  |
Unrealized loss |  |  |  | $ 6,000,000.0 |  |
Investment securities in an unrealized loss position (securities) | Investments |  | 334 |  | 334 |  | 760
Impairment loss |  | $ 0 | $ 0 | $ 0 | $ 0 |
Callable Within One Year |  |  |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |  |  |
Amortized cost of investment securities callable after one year but within five years |  | 160,500,000 |  | 160,500,000 |  |
Fair value of investment securities callable after one year but within five years |  | 160,800,000 |  | 160,800,000 |  |
Callable structured notes amortized costs |  | $ 1,000,000.0 |  | $ 1,000,000.0 |  |

X
- Definition Available-for-Sale and Held-to-Maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions + References No definition available. + Details
Name: | fibk_AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Namespace Prefix: | fibk_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Debt Securities, Held-to-maturity, transfer, Fair value amount + References No definition available. + Details
Name: | fibk_DebtSecuritiesHeldTomaturityTransferFairValueAmount
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Investment Securities,Callable Structured, Amortized Cost Basis + References No definition available. + Details
Name: | fibk_InvestmentSecuritiesCallableStructuredAmortizedCostBasis
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Investment Securities, Primarily Classified as Available-for-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis + References No definition available. + Details
Name: | fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Investment Securities, Primarily Classified as Available-for-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value + References No definition available. + Details
Name: | fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveFairValue
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security, measured at amortized cost, transferred to investment in debt security, measured at fair value, with change in fair value recognized in other comprehensive income (available-for-sale) and net income (trading). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27405-111563 + Details
Name: | us-gaap_DebtSecuritiesHeldToMaturityTransferAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of gain (loss) in accumulated other comprehensive income (AOCI) of derivative hedging forecasted acquisition of investment in debt security measured at amortized cost (held-to-maturity), transferred to investment in debt security measured at fair value with changes in fair value recognized in other comprehensive income (available-for-sale) and net income (trading). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27405-111563 + Details
Name: | us-gaap_DebtSecuritiesHeldToMaturityTransferDerivativeHedgeGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563 + Details
Name: | us-gaap_HeldToMaturitySecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8A -URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284422-111562 + Details
Name: | us-gaap_ImpairmentOfInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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_ScheduleOfHeldToMaturitySecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_InvestmentTypeAxis=fibk_CallableWithinOneYearMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |