Basis of Presentation and Significant Accounting Policies (Policies) |
6 Months Ended |
|---|---|
Jun. 30, 2026 | |
| Accounting Policies [Abstract] | |
| Basis of Presentation | Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States, or GAAP, and applicable rules and regulations of the Securities and Exchange Commission, or SEC, regarding interim financial reporting. The condensed consolidated balance sheet as of June 30, 2026, the condensed consolidated statements of operations, comprehensive loss, and stockholders’ equity for the three and six months ended June 30, 2026 and 2025 and the condensed consolidated statements of cash flows for the six months ended June 30, 2026 and 2025 are unaudited. These unaudited condensed consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position, results of operations and cash flows for the interim period presented. The financial data and other financial information contained in these notes to the condensed consolidated financial statements related to the three month periods are also unaudited. The results of operations for the three and six months ended June 30, 2026 are not necessarily indicative of the results to be expected for the year ending December 31, 2026 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2025 included herein was derived from the audited consolidated financial statements as of that date. These unaudited condensed consolidated financial statements should be read in conjunction with the Company's audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on March 30, 2026. |
| Use of Estimates | Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported expenses during the reporting period. Management evaluates its estimates, including but not limited to the fair value of investments, incremental borrowing rate for operating lease right-of-use assets and liabilities, valuation of deferred tax assets and uncertain tax positions (including valuation allowance), clinical trial accruals and stock-based compensation. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. |
| Principles of Consolidation | Principles of Consolidation The condensed consolidated financial statements include the operations of Annexon, Inc. and its wholly-owned subsidiary and include the results of operations and cash flows of these entities. All intercompany balances and transactions have been eliminated in consolidation. |
| Recently Issued Accounting Pronouncements | Recently Issued Accounting Pronouncements In November 2024, the FASB issued ASU 2024-03, Income Statement-Reporting Comprehensive Income-Expense Disaggregation Disclosures: Disaggregation of Income Statement Expenses, which requires public entities to disclose additional information about specific expense categories in the notes to the financial statements on an interim and annual basis. This guidance is effective for annual reporting periods beginning after December 15, 2026, and interim periods beginning after December 15, 2027. Early adoption is permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. In September 2025, the FASB issued ASU 2025-06, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Targeted Improvements to the Accounting for Internal-Use Software, which is intended to simplify the capitalization guidance for internal-use software by removing references to project stages and clarifying when capitalization of eligible costs is required. ASU 2025-06 is effective for annual periods beginning after December 15, 2027, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. In December 2025, the FASB issued ASU 2025-11, Interim Reporting (Topic 270) Narrow-Scope Improvements. The ASU clarifies and reorganizes interim reporting guidance, including disclosure requirements related to events occurring since the end of the most recent annual reporting period, and improves the presentation and usability of interim financial statement disclosures. The ASU is effective for interim reporting periods within fiscal years beginning after December 15, 2027, with early adoption permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. |