Accounting Principles and Practices	6 Months Ended
Jun. 30, 2026
Accounting Policies [Abstract]
Accounting Principles and Practices	Accounting Principles and Practices New Accounting Pronouncements Accounting Standards Issued But Not Yet Adopted Accounting for and Disclosure of Environmental Credits and Environmental Credit Obligations In May 2026, the FASB issued new accounting guidance under ASC 818, Environmental Credits and Environmental Credit Obligations, which provides guidance on the recognition, measurement, presentation, and disclosure of environmental credits and environmental credit obligations. The guidance is applicable to environmental credits acquired, generated, or received by an entity and requires expanded disclosures related to the nature, intended use, and financial statement impacts. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2027, with early adoption permitted. The Company is currently evaluating the timing of adoption and the impact the guidance will have on the Consolidated Financial Statements and Notes. Accounting for and Disclosure of Software Costs In September 2025, the FASB issued new accounting guidance under ASC 350-40, Intangibles — Goodwill and Other Internal Use Software to modernize the criteria for capitalizing software development costs by removing references to development stages and framework updates to better reflect current software development practices. The new guidance is effective for annual periods beginning after December 15, 2027, with early adoption permitted. The Company is evaluating the impact the new guidance will have on the Consolidated Financial Statements and Notes. Disaggregation of Income Statement Expenses In November 2024, the FASB issued new accounting guidance under ASC 220, Income Statement — Reporting Comprehensive Income, which requires more detailed information about certain expenses in commonly presented expense captions including inventory, employee compensation, depreciation, and amortization. The new guidance also requires disclosure of total selling expenses and, on an annual basis, an entity’s definition of selling expenses. The new guidance is effective for Aon for the year ended December 31, 2027, with early adoption permitted. Entities may apply the new guidance on a prospective basis, with the option for retrospective application. The Company is currently evaluating the transition approach and the impact the guidance will have on the Notes to Consolidated Financial Statements. Securities and Exchange Commission Final Rules The Enhancement and Standardization of Climate-Related Disclosures for Investors In March 2024, the SEC adopted final rules to enhance and standardize climate-related disclosures. The final rules would require the Company to provide certain climate-related information in Item 7, Management’s Discussion and Analysis and the Notes to Consolidated Financial Statements, including regarding material climate-related risks, activities to mitigate or adapt to such risks, information regarding oversight and management of climate-related risks, information on climate-related targets or goals, and disclosure of Scope 1 and 2 GHG emissions. The final rules have been subject to several legal challenges, and the SEC stayed the effectiveness of the final rules in April 2024 pending judicial review. On March 27, 2025, the SEC voted to end its legal defense of the final rules in Court of Appeals for the Eighth Circuit. On September 12, 2025, the Eighth Circuit issued an order holding the consolidated petitions for review in abeyance until the SEC informed the court whether it would reconsider the rules under administrative procedures or whether the SEC would renew its defense of the rules. On May 29, 2026, the SEC proposed to rescind the climate-related disclosure rules in their entirety due to the lack of statutory authority of the SEC. The proposal remains subject to public comment. The Company will continue to monitor developments related to the proposed rescission and any impacts on its disclosure requirements.

In May 2026, the FASB issued new accounting guidance under ASC 818, Environmental Credits and Environmental Credit Obligations, which provides guidance on the recognition, measurement, presentation, and disclosure of environmental credits and environmental credit obligations. The guidance is applicable to environmental credits acquired, generated, or received by an entity and requires expanded disclosures related to the nature, intended use, and financial statement impacts. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2027, with early adoption permitted. The Company is currently evaluating the timing of adoption and the impact the guidance will have on the Consolidated Financial Statements and Notes.

In September 2025, the FASB issued new accounting guidance under ASC 350-40, Intangibles — Goodwill and Other Internal Use Software to modernize the criteria for capitalizing software development costs by removing references to development stages and framework updates to better reflect current software development practices. The new guidance is effective for annual periods beginning after December 15, 2027, with early adoption permitted. The Company is evaluating the impact the new guidance will have on the Consolidated Financial Statements and Notes.

In November 2024, the FASB issued new accounting guidance under ASC 220, Income Statement — Reporting Comprehensive Income, which requires more detailed information about certain expenses in commonly presented expense captions including inventory, employee compensation, depreciation, and amortization. The new guidance also requires disclosure of total selling expenses and, on an annual basis, an entity’s definition of selling expenses. The new guidance is effective for Aon for the year ended December 31, 2027, with early adoption permitted. Entities may apply the new guidance on a prospective basis, with the option for retrospective application. The Company is currently evaluating the transition approach and the impact the guidance will have on the Notes to Consolidated Financial Statements.

In March 2024, the SEC adopted final rules to enhance and standardize climate-related disclosures. The final rules would require the Company to provide certain climate-related information in Item 7, Management’s Discussion and Analysis and the Notes to Consolidated Financial Statements, including regarding material climate-related risks, activities to mitigate or adapt to such risks, information regarding oversight and management of climate-related risks, information on climate-related targets or goals, and disclosure of Scope 1 and 2 GHG emissions. The final rules have been subject to several legal challenges, and the SEC stayed the effectiveness of the final rules in April 2024 pending judicial review. On March 27, 2025, the SEC voted to end its legal defense of the final rules in Court of Appeals for the Eighth Circuit. On September 12, 2025, the Eighth Circuit issued an order holding the consolidated petitions for review in abeyance until the SEC informed the court whether it would reconsider the rules under administrative procedures or whether the SEC would renew its defense of the rules. On May 29, 2026, the SEC proposed to rescind the climate-related disclosure rules in their entirety due to the lack of statutory authority of the SEC. The proposal remains subject to public comment. The Company will continue to monitor developments related to the proposed rescission and any impacts on its disclosure requirements.