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Business and Basis of Presentation Background and Business Description - Additional Information (Details) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2026 USD ($) level3FinancialInstruments | Dec. 31, 2025 USD ($)
Background And Basis Of Presentation [Line Items] |  |
Entity Incorporation, Date of Incorporation | Apr. 29, 1991 |
Number of Admitted Carriers Owned | 5 |
Number of Reportable Segments | 2 |
Short-Term Debt | $ | $ 0 | $ 0
Background and Business Description | 1. BACKGROUND AND BUSINESS DESCRIPTION Business The following description provides an update of Note 1. Background and Business Description and Note 2. Basis of Presentation and Significant Accounting Policies in the Notes to the Consolidated Financial Statements included in Part II, Item 8 in the Company’s Annual Report on Form 10-K for the year ended December 31, 2025, and should be read in conjunction with the complete descriptions provided in the Form 10-K. Capitalized terms used, but not defined herein, and in the other footnotes to the Consolidated Financial Statements included in this Quarterly Report on Form 10-Q shall have the meanings ascribed thereto in the Company’s Annual Report on Form 10-K for the year ended December 31, 2025. Octave Specialty Group, Inc. (“OSG”), headquartered in New York City, is a financial services holding company incorporated in the state of Delaware on April 29, 1991. References to “Octave,” "OSG", the “Company,” “we,” “our,” and “us” are to OSG and its subsidiaries, as the context requires. Octave operates two principal businesses: •Insurance Distribution — Octave's specialty property and casualty ("P&C") insurance underwriting and distribution business, includes Managing General Agents and Underwriters (collectively "MGAs" or "MGA/Us"); an insurance broker; and other distribution, underwriting and related businesses. On October 31, 2025, the Company completed the acquisition of ArmadaCorp Capital, LLC and its subsidiaries (collectively, "ArmadaCorp"), a leading specialty accident and health MGA. Octave's insurance distribution platform operates in the following lines of business: property, niche specialty risk, accident & health, miscellaneous specialty, reinsurance, surety, marine & energy, specialty auto, Excess & Surplus ("E&S") commercial package, professional lines and Directors & Officers ("D&O") . •Specialty Property & Casualty Insurance — Octave's Specialty Property & Casualty Insurance program insurer business currently includes five carriers (collectively, “Everspan”). Everspan carriers have an A.M. Best rating of 'A-' (Excellent), which was affirmed on July 17, 2025. The Company reports these two business operations as segments; see Note 2. Segment Information for further information. |

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- Definition The entire disclosure for the business description and basis of presentation concepts. Business description describes the nature and type of organization including but not limited to organizational structure as may be applicable to holding companies, parent and subsidiary relationships, business divisions, business units, business segments, affiliates and information about significant ownership of the reporting entity. Basis of presentation describes the underlying basis used to prepare the financial statements (for example, US Generally Accepted Accounting Principles, Other Comprehensive Basis of Accounting, IFRS). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483426/235-10-50-4 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 205 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org/205/tableOfContent + Details
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