Nature of Operations and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations and Summary of Significant Accounting Policies	Note 1 — Nature of Operations and Summary of Significant Accounting Policies Nature of Operations The accounting and reporting practices of First Business Financial Services, Inc. (“FBFS” or the “Corporation”), through our wholly-owned subsidiary, First Business Bank (“FBB” or the “Bank”), have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”). FBB operates as a commercial banking institution primarily in Wisconsin and the greater Kansas City metropolitan area. The Bank provides a full range of financial services to businesses, business owners, executives, professionals, and high net worth individuals. FBB also offers bank consulting services to community financial institutions. The Bank is subject to competition from other financial institutions and service providers and is also subject to state and federal regulations. As of March 31, 2024, FBB had the following wholly-owned subsidiaries: First Business Specialty Finance, LLC (“FBSF”), First Madison Investment Corp. (“FMIC”), ABKC Real Estate, LLC (“ABKC”), FBB Real Estate 2, LLC (“FBB RE 2”), Mitchell Street Apartments Investment, LLC (“Mitchell Street”), and FBB Tax Credit Investment, LLC (“FBB Tax Credit”). Basis of Presentation The accompanying unaudited Consolidated Financial Statements were prepared in accordance with GAAP and the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements and should be read in conjunction with the Corporation’s Consolidated Financial Statements and footnotes thereto included in the Corporation’s Annual Report on Form 10-K for the year ended December 31, 2023. The unaudited Consolidated Financial Statements include the accounts of the Corporation and its wholly-owned subsidiaries. All significant intercompany balances and transactions have been eliminated in consolidation. Management of the Corporation is required to make estimates and assumptions which affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements as well as reported amounts of revenues and expenses during the reporting period. Actual results could differ significantly from those estimates. Material estimates that could significantly change in the near-term include the value of securities and interest rate swaps, level of the allowance for credit losses, lease residuals, property under operating leases, goodwill, and income taxes. The results of operations for the three months ended March 31, 2024, are not necessarily indicative of results that may be expected for any other interim period or the entire fiscal year ending December 31, 2024. Certain amounts in prior periods may have been reclassified to conform to the current presentation. Subsequent events have been evaluated through the date of the issuance of the unaudited Consolidated Financial Statements. No significant subsequent events have occurred through this date requiring adjustment to the financial statements or disclosures. The Corporation has not changed its significant accounting and reporting policies from those disclosed in the Corporation’s Form 10-K for the year ended December 31, 2023. Recent Accounting Pronouncements In March 2023, the Financial Accounting Standards Board issued Accounting Standards Update No. 2023-02 “Investments-Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method (a consensus of the Emerging Issues Task Force).” The amendments in this Update permit reporting entities to elect to account for their tax equity investments, regardless of the program from which the income tax credits are received, using the proportional amortization method if certain conditions are met. A reporting entity may make an accounting policy election to apply the proportional amortization method on a tax-credit-program-by-tax-credit-program basis rather than electing to apply the proportional amortization method at the reporting entity level or to individual investments. The ASU is effective for annual and interim periods beginning after December 15, 2023. The Corporation adopted the standard effective January 1, 2024. The implementation did not have a material effect on the Consolidated Financial Statements. In October 2023, the FASB issued ASU No. 2023-06, “Disclosure Improvements: Codification Amendments in Response to the SEC’s Disclosure Update and Simplification Initiative.” This update is intended to improve the relevance and usefulness of financial information for investors and other users by incorporating certain SEC disclosure requirements into the FASB Accounting Standards Codification. This update is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2023. The adoption of this standard did not have a material effect on the Consolidated Financial Statements. In November 2023, the FASB issued ASU No. 2023-07, “Segment Reporting (Topic 820): Improvements to Reportable Segment Disclosures.” This update is intended to improve reportable segment disclosure requirements, primarily through enhanced disclosures about significant segment expenses. This update is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2024. The Corporation is assessing the impact of the standard. In December 2023, the FASB issued ASU No. 2023-09, “Income Taxes (Topic 740): Improvements to Income Tax Disclosures.” This update enhances the transparency and decision usefulness of income tax disclosures by providing better information regarding exposure to potential changes in jurisdictional tax legislation and related forecasting and cash flow opportunities. This update is effective for fiscal years beginning after December 15, 2024. The Corporation is assessing the impact of the standard.