Basis of Presentation	6 Months Ended
Jun. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	In the opinion of management, the accompanying unaudited consolidated financial statements of First Interstate BancSystem, Inc., and its consolidated subsidiaries, including its wholly-owned subsidiary, First Interstate Bank (“FIB”) (collectively, the “Company”) contain all adjustments (all of which are of a normal recurring nature) necessary to present fairly the financial position of the Company at June 30, 2023 and December 31, 2022, the results of operations, changes in stockholders’ equity, and cash flows for each of the three and the six month periods ended June 30, 2023 and 2022, in conformity with U.S. generally accepted accounting principles (“GAAP”). The balance sheet information at December 31, 2022 is derived from the audited consolidated financial statements. Certain reclassifications, none of which were material, have been made to conform the Company’s prior year financial statements to the June 30, 2023 presentation. These reclassifications did not change previously reported net loss or stockholders’ equity. These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and related notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022, which includes a description of significant accounting policies. Operating results for the three and the six months ended June 30, 2023 are not necessarily indicative of the results that may be expected for the year ending December 31, 2023. The below detailed discussion updates the accounting policies included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022. Change in Par Value Unless otherwise noted, all capital values, and share and per share amounts, included in the consolidated financial statements have been retroactively adjusted to account for the change in the Company’s common stock par value from no par value per share to $0.00001 per value per share, which change became effective on May 25, 2023 in connection with the re-domestication of the Company from the State of Montana to the State of Delaware. During the second quarter of 2023, the Company transitioned its Current Expected Credit Loss accounting standard (“CECL”), or ASC 326, model. The new model utilizes fewer portfolio loss segments than the old model due to limited observations of data within certain portfolio loss segments under the old model. The new model calculates historical loss rates by averaging quarterly net charge-offs for each loss segment. The loss rates are macroeconomic-conditioned and applied to loan-level expected cash flows based on contractual repayment terms while considering prepayment, utilization, interest rate, and probability of default assumptions. This change did not result in a material impact to the Company’s financial statements. Allowance for Credit Losses - Loans held for investment The allowance for credit losses is a valuation account that is deducted from the loans’ amortized cost basis to present the net amount expected to be collected over the life of the loans. Loans are charged off against the allowance when management confirms that a loan balance is uncollectable. The Company applies recoveries when received and has elected not to forecast recoveries. Management estimates the allowance balance using relevant available information, from internal and external sources, relating to past events, current conditions, and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. Adjustments to historical loss information are made for differences in current loan-specific risk characteristics such as differences in underwriting standards, portfolio mix and trends, asset quality, or term as well as for changes in environmental and economic conditions, such as changes in unemployment rates, property values, or other relevant factors. The allowance for credit losses is measured on a collective basis when similar risk characteristics exist. Annualized loss rates are recalculated quarterly, with subsequent recoveries captured in the quarter a loan was charged off, and are averaged across a look back period from 2009 to the current period. Expected future principal and interest cash flows are calculated using contractual repayment terms while considering prepayment, utilization, interest rate, and probability of default assumptions. Macroeconomic models calculate segment-specific multipliers using third party forecast data. The multipliers adjust the annual loss rates to the level expected under the economic conditions over the 2-year forecast period, followed by a 1-year straight-line reversion to the unadjusted historical average loss rates. The unadjusted loss rates then apply for the remaining life of the loan. Estimated losses are totaled and aggregated to the portfolio segment level. The Company segments the loan portfolio into pools based on the following risk characteristics: financial asset type, collateral type, loan structure, credit characteristics, outstanding loan balances, contractual terms and prepayment assumptions, utilization assumptions, industry of borrower and concentrations, and historical or expected credit loss. The Company has identified the following portfolio segments: Real Estate loans. Include commercial real estate loans which are non-farm, non-residential real estate loans generally secured by first liens on income-producing real estate and generally mature in less than 10 years, construction development loans which are primarily to commercial builders for residential lot development and the construction of single-family residences and commercial real estate properties, commercial construction loans which are primarily made to commercial builders for the development of commercial real estate properties, construction and development loans which are generally underwritten pursuant to credit worthiness or pre-qualification for permanent financing. During the construction phase the borrower pays interest only. Construction and development loans generally transition to a permanent real estate loan or otherwise mature in three years or less. Real estate loans also include agricultural real estate loans that generally mature in ten years or less, secured by farmland or ranchland consisting of short, intermediate, and long-term structures to experienced agriculturalists who have demonstrated management capabilities, established production and historical financial performance and consumer home equity and home equity lines of credit (“HELOC”) that are secured by residential property that generally mature in 25 years or less, and residential loans which are loans to finance the purchase or refinance of residential property which are typically secured by first liens or are construction loans to commercial builders or owner occupants for the construction of single-family residences. Residential 1-4 family loans generally mature within 15 years but could mature up to 30 years. Construction loans are generally underwritten pursuant to credit worthiness or pre-qualification for permanent financing. During the construction phase the borrower pays interest only. Residential construction loans generally transition to a permanent residential loan or otherwise mature in two years or less. Consumer loans. Include indirect, direct and advanceline, and credit cards. Indirect are loan contracts advanced for the purchase of automobiles, boats, and other consumer goods from the consumer product dealer networks within the market areas we serve. Indirect dealer loans are generally secured by automobiles, recreational vehicles, boats, and other types of personal property and are made on an installment basis. Consumer indirect line loans generally mature in seven years or less. Consumer direct and advance line loans are originated for a variety of purposes including the purchase of automobiles, boats and other consumer goods, home improvements, medical expenses, vehicle repairs, debt consolidation, and planned expenses in addition to the purchase of automobiles, boats, and other consumer goods. Consumer direct and advance line loans generally mature in seven years or less. Consumer credit card loans are lines of credit offered to clients in our market areas that are generally floating rate loans and include both unsecured and secured lines. Consumer credit card loans generally do not have stated maturities but are reviewed periodically and are unconditionally cancellable. Commercial loans. Include commercial and industrial loans through a mix of variable and fixed rate commercial loans, including loans to finance showroom floor inventories and other loans for commercial purposes that are secured by 1-4 family residential property. These loans are typically made to small and medium-sized manufacturing, wholesale, retail, and service businesses for working capital needs and business expansions. Floor plan loans and commercial purpose loans secured by 1-4 family generally mature in seven years or less. Commercial loans also include secured and unsecured lines of credit, business credit cards, and loans with maturities of five years or less. Outstanding balances on these commercial loans tend to be cyclical in nature. These loans are generally made with business operations as the primary source of repayment, and are typically collateralized by inventory, accounts receivable, equipment, and/or personal guarantees. Agricultural loans. Agricultural loans generally consist of short and medium-term loans and lines of credit that are primarily used for crops, livestock, equipment, and general operations. Agricultural loans are ordinarily secured by assets such as livestock or equipment and are repaid from the operations of the farm or ranch. Agricultural loans generally have maturities of seven years or less, with operating lines for one production season. Contractual Term. Expected credit losses are estimated over the contractual term of the loans, adjusted for expected prepayments, defaults, interest rates, and utilization rates. The contractual term excludes expected extensions, renewals, and modifications unless either management has a reasonable expectation at the reporting date that a modification to borrowers experiencing financial difficulty will be executed or the extension or renewal options are included in the original or modified contract at the reporting date and are not unconditionally cancellable by the Company. A loan for which the terms have been modified resulting in a concession, and for which the borrower is experiencing financial difficulties, is considered to be a modification to borrowers experiencing financial difficulty. The allowance for credit loss on a modification to borrowers experiencing financial difficulty is measured using the same method as all other loans held for investment, except when the loan is individually assessed for credit loss. Allowance for Credit Losses on Off-Balance Sheet Credit Exposures. The Company estimates expected credit losses over the contractual period in which the Company is exposed to credit risk via a contractual obligation to extend credit unless that obligation is unconditionally cancellable by the Company. Management considers our unused credit card lines and federal fund lines, extended to others, to be unconditionally cancellable. The allowance for credit losses on off-balance sheet credit exposures is adjusted as a provision for credit loss expense. The estimate considers the likelihood that funding will occur and an estimate of expected credit losses on commitments expected to be funded over the estimated life. The Company has identified commitments to extend credit and standby letters of credit determined not to be unconditionally cancellable as categories with off-balance sheet credit exposures and uses the commitment balance, expected loss rate, expected cash flows, and utilization rate as primary assumptions to develop the allowance for credit losses on those exposures. The utilization rate represents management’s best estimate of the probability that the unfunded portion of the commitment will be funded given existing economic conditions.

In the opinion of management, the accompanying unaudited consolidated financial statements of First Interstate BancSystem, Inc., and its consolidated subsidiaries, including its wholly-owned subsidiary, First Interstate Bank (“FIB”) (collectively, the “Company”) contain all adjustments (all of which are of a normal recurring nature) necessary to present fairly the financial position of the Company at June 30, 2023 and December 31, 2022, the results of operations, changes in stockholders’ equity, and cash flows for each of the three and the six month periods ended June 30, 2023 and 2022, in conformity with U.S. generally accepted accounting principles (“GAAP”). The balance sheet information at December 31, 2022 is derived from the audited consolidated financial statements. Certain reclassifications, none of which were material, have been made to conform the Company’s prior year financial statements to the June 30, 2023 presentation. These reclassifications did not change previously reported net loss or stockholders’ equity.

These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and related notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022, which includes a description of significant accounting policies. Operating results for the three and the six months ended June 30, 2023 are not necessarily indicative of the results that may be expected for the year ending December 31, 2023.

Unless otherwise noted, all capital values, and share and per share amounts, included in the consolidated financial statements have been retroactively adjusted to account for the change in the Company’s common stock par value from no par value per share to $0.00001 per value per share, which change became effective on May 25, 2023 in connection with the re-domestication of the Company from the State of Montana to the State of Delaware.

During the second quarter of 2023, the Company transitioned its Current Expected Credit Loss accounting standard (“CECL”), or ASC 326, model. The new model utilizes fewer portfolio loss segments than the old model due to limited observations of data within certain portfolio loss segments under the old model. The new model calculates historical loss rates by averaging quarterly net charge-offs for each loss segment. The loss rates are macroeconomic-conditioned and applied to loan-level expected cash flows based on contractual repayment terms while considering prepayment, utilization, interest rate, and probability of default assumptions. This change did not result in a material impact to the Company’s financial statements.

The allowance for credit losses is a valuation account that is deducted from the loans’ amortized cost basis to present the net amount expected to be collected over the life of the loans. Loans are charged off against the allowance when management confirms that a loan balance is uncollectable. The Company applies recoveries when received and has elected not to forecast recoveries.

Management estimates the allowance balance using relevant available information, from internal and external sources, relating to past events, current conditions, and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. Adjustments to historical loss information are made for differences in current loan-specific risk characteristics such as differences in underwriting standards, portfolio mix and trends, asset quality, or term as well as for changes in environmental and economic conditions, such as changes in unemployment rates, property values, or other relevant factors.

Annualized loss rates are recalculated quarterly, with subsequent recoveries captured in the quarter a loan was charged off, and are averaged across a look back period from 2009 to the current period. Expected future principal and interest cash flows are calculated using contractual repayment terms while considering prepayment, utilization, interest rate, and probability of default assumptions. Macroeconomic models calculate segment-specific multipliers using third party forecast data. The multipliers adjust the annual loss rates to the level expected under the economic conditions over the 2-year forecast period, followed by a 1-year straight-line reversion to the unadjusted historical average loss rates. The unadjusted loss rates then apply for the remaining life of the loan. Estimated losses are totaled and aggregated to the portfolio segment level.

The Company segments the loan portfolio into pools based on the following risk characteristics: financial asset type, collateral type, loan structure, credit characteristics, outstanding loan balances, contractual terms and prepayment assumptions, utilization assumptions, industry of borrower and concentrations, and historical or expected credit loss.

Real Estate loans. Include commercial real estate loans which are non-farm, non-residential real estate loans generally secured by first liens on income-producing real estate and generally mature in less than 10 years, construction development loans which are primarily to commercial builders for residential lot development and the construction of single-family residences and commercial real estate properties, commercial construction loans which are primarily made to commercial builders for the development of commercial real estate properties, construction and development loans which are generally underwritten pursuant to credit worthiness or pre-qualification for permanent financing. During the construction phase the borrower pays interest only. Construction and development loans generally transition to a permanent real estate loan or otherwise mature in three years or less.

Real estate loans also include agricultural real estate loans that generally mature in ten years or less, secured by farmland or ranchland consisting of short, intermediate, and long-term structures to experienced agriculturalists who have demonstrated management capabilities, established production and historical financial performance and consumer home equity and home equity lines of credit (“HELOC”) that are secured by residential property that generally mature in 25 years or less, and residential loans which are loans to finance the purchase or refinance of residential property which are typically secured by first liens or are construction loans to commercial builders or owner occupants for the construction of single-family residences. Residential 1-4 family loans generally mature within 15 years but could mature up to 30 years. Construction loans are generally underwritten pursuant to credit worthiness or pre-qualification for permanent financing. During the construction phase the borrower pays interest only. Residential construction loans generally transition to a permanent residential loan or otherwise mature in two years or less.

Consumer loans. Include indirect, direct and advanceline, and credit cards. Indirect are loan contracts advanced for the purchase of automobiles, boats, and other consumer goods from the consumer product dealer networks within the market areas we serve. Indirect dealer loans are generally secured by automobiles, recreational vehicles, boats, and other types of personal property and are made on an installment basis. Consumer indirect line loans generally mature in seven years or less. Consumer direct and advance line loans are originated for a variety of purposes including the purchase of automobiles, boats and other consumer goods, home improvements, medical expenses, vehicle repairs, debt consolidation, and planned expenses in addition to the purchase of automobiles, boats, and other consumer goods. Consumer direct and advance line loans generally mature in seven years or less. Consumer credit card loans are lines of credit offered to clients in our market areas that are generally floating rate loans and include both unsecured and secured lines. Consumer credit card loans generally do not have stated maturities but are reviewed periodically and are unconditionally cancellable.

Commercial loans. Include commercial and industrial loans through a mix of variable and fixed rate commercial loans, including loans to finance showroom floor inventories and other loans for commercial purposes that are secured by 1-4 family residential property. These loans are typically made to small and medium-sized manufacturing, wholesale, retail, and service businesses for working capital needs and business expansions. Floor plan loans and commercial purpose loans secured by 1-4 family generally mature in seven years or less. Commercial loans also include secured and unsecured lines of credit, business credit cards, and loans with maturities of five years or less. Outstanding balances on these commercial loans tend to be cyclical in nature. These loans are generally made with business operations as the primary source of repayment, and are typically collateralized by inventory, accounts receivable, equipment, and/or personal guarantees.

Agricultural loans. Agricultural loans generally consist of short and medium-term loans and lines of credit that are primarily used for crops, livestock, equipment, and general operations. Agricultural loans are ordinarily secured by assets such as livestock or equipment and are repaid from the operations of the farm or ranch. Agricultural loans generally have maturities of seven years or less, with operating lines for one production season.

Contractual Term. Expected credit losses are estimated over the contractual term of the loans, adjusted for expected prepayments, defaults, interest rates, and utilization rates. The contractual term excludes expected extensions, renewals, and modifications unless either management has a reasonable expectation at the reporting date that a modification to borrowers experiencing financial difficulty will be executed or the extension or renewal options are included in the original or modified contract at the reporting date and are not unconditionally cancellable by the Company.

A loan for which the terms have been modified resulting in a concession, and for which the borrower is experiencing financial difficulties, is considered to be a modification to borrowers experiencing financial difficulty. The allowance for credit loss on a modification to borrowers experiencing financial difficulty is measured using the same method as all other loans held for investment, except when the loan is individually assessed for credit loss.

Allowance for Credit Losses on Off-Balance Sheet Credit Exposures. The Company estimates expected credit losses over the contractual period in which the Company is exposed to credit risk via a contractual obligation to extend credit unless that obligation is unconditionally cancellable by the Company. Management considers our unused credit card lines and federal fund lines, extended to others, to be unconditionally cancellable.

The allowance for credit losses on off-balance sheet credit exposures is adjusted as a provision for credit loss expense. The estimate considers the likelihood that funding will occur and an estimate of expected credit losses on commitments expected to be funded over the estimated life.