Nature of Operations	9 Months Ended
Sep. 30, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations Marsh & McLennan Companies, Inc., and its consolidated subsidiaries (the "Company" or "Marsh") is a global professional services firm in the areas of risk, reinsurance and capital, people and investments, and management consulting. Effective January 14, 2026, the Company updated its brand name from Marsh McLennan to Marsh and the brand names of Marsh and Oliver Wyman Group businesses to Marsh Risk ("Risk") and Marsh Management Consulting ("Management Consulting"), respectively. As a result of the Company's previously announced brand change, results that were previously reported under our Guy Carpenter business will now be reported as "Reinsurance" and results that were previously reported as Mercer will now be reported as "People and Investments". Within People and Investments, results for Wealth will now be reported as "Investments and Retirement", Health will now be reported as "Health and Benefits" and Career will now be reported as "Workforce and Rewards". Within Marsh Risk, the Company updated the brand name of Marsh McLennan Agency to Marsh Agency. References to the Company and its businesses in the consolidated financial statements reflect these changes. The changes to the brand names had no impact on the Company's operating and reporting segments. The Company is organized based on the different services that it offers. Under this structure, the Company’s two business segments are Risk and Insurance Services and Consulting. The Risk and Insurance Services segment ("RIS") includes risk management activities and insurance/reinsurance broking and services conducted through Risk and Reinsurance. Risk is an insurance broker and risk advisor, offering risk management, insurance broking, insurance program management, risk consulting, analytical modeling and alternative risk financing services, to a wide range of businesses, government entities, professional service organizations and individuals. Reinsurance, the Company's reinsurance intermediary and advisor, provides specialized reinsurance broking, strategic advisory and actuarial services, and analytics solutions. The Consulting segment includes investments and retirement, health and benefits, and workforce and rewards solutions and products, and specialized management, strategic, economic and brand consulting services conducted through People and Investments and Management Consulting. People and Investments helps clients achieve their investment objectives, shape the future of work, and improve health and retirement outcomes through advice, solutions, and products. Management Consulting offers management consulting and advisory services across various industries.

Marsh & McLennan Companies, Inc., and its consolidated subsidiaries (the "Company" or "Marsh") is a global professional services firm in the areas of risk, reinsurance and capital, people and investments, and management consulting.

Effective January 14, 2026, the Company updated its brand name from Marsh McLennan to Marsh and the brand names of Marsh and Oliver Wyman Group businesses to Marsh Risk ("Risk") and Marsh Management Consulting ("Management Consulting"), respectively. As a result of the Company's previously announced brand change, results that were previously reported under our Guy Carpenter business will now be reported as "Reinsurance" and results that were previously reported as Mercer will now be reported as "People and Investments". Within People and Investments, results for Wealth will now be reported as "Investments and Retirement", Health will now be reported as "Health and Benefits" and Career will now be reported as "Workforce and Rewards". Within Marsh Risk, the Company updated the brand name of Marsh McLennan Agency to Marsh Agency. References to the Company and its businesses in the consolidated financial statements reflect these changes.

The Risk and Insurance Services segment ("RIS") includes risk management activities and insurance/reinsurance broking and services conducted through Risk and Reinsurance. Risk is an insurance broker and risk advisor, offering risk management, insurance broking, insurance program management, risk consulting, analytical modeling and alternative risk financing services, to a wide range of businesses, government entities, professional service organizations and individuals. Reinsurance, the Company's reinsurance intermediary and advisor, provides specialized reinsurance broking, strategic advisory and actuarial services, and analytics solutions.

The Consulting segment includes investments and retirement, health and benefits, and workforce and rewards solutions and products, and specialized management, strategic, economic and brand consulting services conducted through People and Investments and Management Consulting. People and Investments helps clients achieve their investment objectives, shape the future of work, and improve health and retirement outcomes through advice, solutions, and products. Management Consulting offers management consulting and advisory services across various industries.