Nature of Operations and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations and Summary of Significant Accounting Policies	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 private wealth management services and bank consulting services. The Bank is subject to competition from other financial institutions and service providers, and is also subject to state and federal regulations. FBB has 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”), Rimrock Road Investment Fund, LLC (“Rimrock Road”), BOC Investment, LLC (“BOC”), Mitchell Street Apartments Investment, LLC (“Mitchell Street”), and FBB Tax Credit Investment, LLC (“FBB Tax Credit”). FMIC is located in and was formed under the laws of the state of Nevada. 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, 2020. 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. In accordance with the provisions of Accounting Standards Codification (“ASC”) Topic 810, the Corporation’s ownership interest in FBFS Statutory Trust II (“Trust II”) has not been consolidated into the financial statements. 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 loan and lease losses, lease residuals, property under operating leases, goodwill, level of the Small Business Administration (“SBA”) recourse reserve, and income taxes. The results of operations for the three and nine months ended September 30, 2021, are not necessarily indicative of results that may be expected for any other interim period or the entire fiscal year ending December 31, 2021. 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, 2020. Recent Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (FASB) issued ASU No. 2016-13, “Financial Instruments- Credit Losses (Topic 326),” which is often referred to as CECL. The ASU replaces the incurred loss impairment methodology for recognizing credit losses with a methodology that reflects all expected credit losses. The ASU also requires consideration of a broader range of information to inform credit loss estimates, including such factors as past events, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount. The amendments affect loans, debt securities, trade receivables, net investments in leases, off-balance sheet credit exposures, and any other financial asset not excluded from the scope under which the Corporation has the contractual right to receive cash. Entities will apply the amendments in the ASU through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is effective. In November 2019, the FASB issued ASU No. 2019-10, “Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842).” The ASU delays the effective date for the credit losses standard from January 2020 to January 2023 for certain entities, including certain Securities and Exchange Commission filers, public business entities, and private companies. As a smaller reporting company, the Corporation is eligible for the delay and will be deferring adoption. The Corporation has established a cross-functional committee and has implemented a third-party software solution to assist with the adoption of the standard. Management has gathered all necessary data and reviewed potential methods to calculate the expected credit losses. Management is currently calculating sample expected loss computations and developing the allowance methodology and assumptions that will be used under the new standard. Management will continue to progress on its implementation project plan and improve the Corporation’s approach throughout the deferral period. In March 2020, the FASB issued ASU No. 2020-04 “Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting.” These amendments provide temporary optional guidance to ease the potential burden in accounting for reference rate reform. The ASU provides optional expedients and exceptions for applying generally accepted accounting principles to contract modifications and hedging relationships, subject to meeting certain criteria, that reference LIBOR or another reference rate expected to be discontinued. It is intended to help stakeholders during the global market-wide reference rate transition period. In January 2021, the FASB issued ASU 2021-01 which clarifies that certain optional expedients and exceptions in Topic 848 for contract modifications and hedge accounting apply to derivatives that are affected by the discounting transition. The guidance is effective for all entities as of March 12, 2020 through December 31, 2022. The Corporation continues to implement its transition plan toward cessation of LIBOR and the modification of its loans and other financial instruments with attributes that are either directly or indirectly influenced by LIBOR. The Corporation expects to utilize the LIBOR transition relief allowed under ASU 2020-04 and ASU 2020-01, as applicable, and does not expect such adoption to have a material impact on its accounting and disclosures. The Corporation will continue to assess the impact as the reference rate transition occurs over the next two years. In August 2021, the FASB issued Accounting Standards Update (ASU) No. 2021-06 “Presentation of Financial Statements (Topic 205), Financial Services-Depository and Lending (Topic 942), and Financial Services-Investment Companies (Topic 946): Amendments to SEC Paragraphs Pursuant to SEC Final Rule Releases No. 33-10786, Amendments to Financial Disclosures about Acquired and Disposed Businesses, and No. 33-10835, Update of Statistical Disclosures for Bank and Savings and Loan Registrants.” This ASU amends the SEC sections of the Codification related to Final Rule Releases No. 33-10786, Amendments to Financial Disclosures about Acquired and Disposed Businesses, and No. 33-10835, Update to Statistical Disclosures for Bank and Savings and Loan Registrants. The guidance is effective upon its addition to the FASB codification. The Corporation is assessing the impact of ASU 2021-06 and its impact on its disclosures.