Basis of Presentation	6 Months Ended
Jun. 30, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	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, 2025 and December 31, 2024, the results of operations, changes in stockholders’ equity, and cash flows for each of the three and the six months ended June 30, 2025 and 2024, in conformity with U.S. generally accepted accounting principles (“GAAP”). The balance sheet information at December 31, 2024 is derived from audited consolidated financial statements. The unaudited consolidated financial statements have been prepared in conformity with the required interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X; and therefore, do not include all of the information and footnotes required by GAAP for a complete set of financial statements. Certain reclassifications, none of which were material, have been made to conform the Company’s prior year financial statements to the financial statements as of June 30, 2025. These reclassifications did not change previously reported net income, financial condition, cash flows, or stockholders’ equity. The Company has identified one reporting unit and one operating segment, community banking, which encompasses commercial and consumer banking services to serve a similar base of clients utilizing company-wide offerings of similar products and services managed through similar processes and platforms offered to individuals, businesses, municipalities, and other entities, whose primary sources of revenue are interest income on loans, investment securities and other interest-earning assets and fee income earned on its products and services. Its expenses consist of interest expense on deposits and borrowed funds, provision for credit losses, other operating expenses and income taxes, as further described below. The Company manages its business activities on a consolidated basis. The Company’s chief operating decision maker (“CODM”) is its Chief Executive Officer who is charged with management of the Company and is responsible for the evaluation of operating performance and decision making about the allocation of capital and resources. The CODM regularly assesses performance of the single operating and reporting segment and decides how to allocate resources based on net income calculated on the same basis as is net income reported in the Company’s consolidated statements of income. The CODM is also regularly provided with expense information at a level consistent with that disclosed in the Company’s consolidated statements of income. The Company’s principal expenses include: (i) interest expense on deposit accounts and other borrowings; (ii) salaries and employee benefits; (iii) information technology and communication costs primarily associated with maintaining loan and deposit functions; (iv) furniture, equipment, and occupancy expenses for maintaining our facilities; (v) professional fees, including FDIC insurance assessments; (vi) income tax expense; (vii) provisions for credit losses; (viii) intangible amortization; (ix) other real estate owned expenses; and (x) other segment expenses including legal expenses, advertising and promotion, donations, credit card rewards expense, fees associated with originating and closing loans, insurance, and other expenses necessary to support our employees and service our clients. See the consolidated financial statements for other financial information regarding the Company’s operating segment. The accounting policies of the segment are the same as those described in "Note 1 - Summary of Significant Accounting Policies" of the Company's Annual Report on Form 10-K for the year ended December 31, 2024. 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, 2024, which includes a description of significant accounting policies. Operating results for the three and the six months ended June 30, 2025 are not necessarily indicative of the results that may be expected for the year ending December 31, 2025.

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, 2025 and December 31, 2024, the results of operations, changes in stockholders’ equity, and cash flows for each of the three and the six months ended June 30, 2025 and 2024, in conformity with U.S. generally accepted accounting principles (“GAAP”). The balance sheet information at December 31, 2024 is derived from audited consolidated financial statements. The unaudited consolidated financial statements have been prepared in conformity with the required interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X; and therefore, do not include all of the information and footnotes required by GAAP for a complete set of financial statements. Certain reclassifications, none of which were material, have been made to conform the Company’s prior year financial statements to the financial statements as of June 30, 2025. These reclassifications did not change previously reported net income, financial condition, cash flows, or stockholders’ equity.

The Company has identified one reporting unit and one operating segment, community banking, which encompasses commercial and consumer banking services to serve a similar base of clients utilizing company-wide offerings of similar products and services managed through similar processes and platforms offered to individuals, businesses, municipalities, and other entities, whose primary sources of revenue are interest income on loans, investment securities and other interest-earning assets and fee income earned on its products and services. Its expenses consist of interest expense on deposits and borrowed funds, provision for credit losses, other operating expenses and income taxes, as further described below. The Company manages its business activities on a consolidated basis.

The CODM regularly assesses performance of the single operating and reporting segment and decides how to allocate resources based on net income calculated on the same basis as is net income reported in the Company’s consolidated statements of income. The CODM is also regularly provided with expense information at a level consistent with that disclosed in the Company’s consolidated statements of income.

The Company’s principal expenses include: (i) interest expense on deposit accounts and other borrowings; (ii) salaries and employee benefits; (iii) information technology and communication costs primarily associated with maintaining loan and deposit functions; (iv) furniture, equipment, and occupancy expenses for maintaining our facilities; (v) professional fees, including FDIC insurance assessments; (vi) income tax expense; (vii) provisions for credit losses; (viii) intangible amortization; (ix) other real estate owned expenses; and (x) other segment expenses including legal expenses, advertising and promotion, donations, credit card rewards expense, fees associated with originating and closing loans, insurance, and other expenses necessary to support our employees and service our clients. See the consolidated financial statements for other financial information regarding the Company’s operating segment.

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, 2024, which includes a description of significant accounting policies. Operating results for the three and the six months ended June 30, 2025 are not necessarily indicative of the results that may be expected for the year ending December 31, 2025.