XML 114 R71.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |  |  |
Gross realized gains | $ 0.0 |  | $ 1.1
Gross realized losses |  | $ 0.0 | $ 0.4
Schedule of Available-for-sale Securities |  |  |
Less than 12 Months | 925.4 | 1,123.2 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5.4 | 22.6 |
12 Months or More | 202.5 | 779.0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1.8 | 14.5 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,127.9 | 1,902.2 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 7.2 | 37.1 |
US Government Agencies Debt Securities [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Less than 12 Months | 185.3 | 363.1 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.8 | 7.9 |
12 Months or More | 0.0 | 154.5 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | 2.3 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 185.3 | 517.6 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 0.8 | 10.2 |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Less than 12 Months | 740.1 | 735.2 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 4.6 | 14.5 |
12 Months or More | 155.9 | 503.7 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1.4 | 9.6 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 896.0 | 1,238.9 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 6.0 | 24.1 |
State, county and municipal securities |  |  |
Schedule of Available-for-sale Securities |  |  |
General obligation securities, amortized cost | 45.9 |  |
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming |  |  |
Schedule of Available-for-sale Securities |  |  |
General obligation securities, amortized cost | 28.0 |  |
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Less than 12 Months | 0.0 | 0.0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | 0.0 |
12 Months or More | 46.6 | 69.4 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.4 | 1.8 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 46.6 | 69.4 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 0.4 | 1.8 |
Corporate Debt Securities [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Less than 12 Months |  | 24.9 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 0.2 |
12 Months or More |  | 51.4 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0.8 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 76.3 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | $ 1.0 |

X
- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

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

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

X
- Details
Name: | srt_StatementGeographicalAxis=fibk_IdahoMontanaOregonSouthDakotaWashingtonandWyomingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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