Nature of Operations	3 Months Ended
Mar. 31, 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 and Marsh Management Consulting, respectively. References to the Company and its businesses in the consolidated financial statements reflect these changes. Mercer and Guy Carpenter will continue to report under their current brands through a transition period. 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 Marsh Risk and Guy Carpenter. Marsh 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. Guy Carpenter, the Company's reinsurance intermediary and advisor, provides specialized reinsurance broking, strategic advisory and actuarial services, and analytics solutions. The Consulting segment includes health, wealth and career advice, solutions and products, and specialized management, strategic, economic and brand consulting services conducted through Mercer and Marsh Management Consulting. Mercer delivers advice, solutions and products that help organizations meet the health, wealth and career needs of a changing workforce. Marsh Management Consulting offers management consulting and advisory services across various industries.

Nature of Operations

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

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 and Marsh Management Consulting, respectively. References to the Company and its businesses in the consolidated financial statements reflect these changes. Mercer and Guy Carpenter will continue to report under their current brands through a transition period.

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 Marsh Risk and Guy Carpenter. Marsh 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. Guy Carpenter, the Company's reinsurance intermediary and advisor, provides specialized reinsurance broking, strategic advisory and actuarial services, and analytics solutions.

The Consulting segment includes health, wealth and career advice, solutions and products, and specialized management, strategic, economic and brand consulting services conducted through Mercer and Marsh Management Consulting. Mercer delivers advice, solutions and products that help organizations meet the health, wealth and career needs of a changing workforce. Marsh Management Consulting offers management consulting and advisory services across various industries.