Nature of Operations	3 Months Ended
Nature of Operations	Mar. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations Marsh & McLennan Companies, Inc. (the "Company"), a global professional services firm, 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") provides risk management solutions (risk advice, risk transfer and risk control and mitigation) as well as insurance and reinsurance broking and services for businesses, public entities, insurance companies, associations, professional services organizations, and private clients. The Company conducts business in this segment through Marsh and Guy Carpenter. Marsh provides data-driven risk advisory services and solutions to commercial and consumer clients. Guy Carpenter develops advanced risk, reinsurance and capital strategies that help clients grow profitably and identify and capitalize on emerging opportunities. The Company conducts business in its Consulting segment through Mercer and Oliver Wyman Group. Mercer delivers advice and solutions that help organizations create a dynamic world of work, shape retirement and investment outcomes, and unlock health and well being for a changing workforce. Oliver Wyman Group serves as critical strategic, economic and brand advisor to private sector and governmental clients. Deconsolidation of Russia On February 24, 2022, Russian forces launched a military invasion of Ukraine. In response, the United States, the European Union, United Kingdom and other governments have imposed significant economic sanctions on Russia, and Russia has responded with counter-sanctions. The war in Ukraine has disrupted international commerce and the global economy. On March 10, 2022, the Company announced it would exit its businesses in Russia and transfer ownership of its Russian businesses to local management who will operate independently in the Russian market. In the first quarter of 2022, the Company concluded that it does not meet the accounting criteria for control over its wholly-owned Russian businesses due to the evolving trade and economic sanctions against Russia and related Russian counter-sanctions. These sanctions include restrictions on payments to and from Russian companies and reduced currency access through official exchange markets that have significantly impacted the Company's ability to effectively manage and operate its Russian businesses. For the three months ended March 31, 2022, the Company recorded a loss of $52 million on the deconsolidation of the Russian businesses and other related charges. Refer to Note 8, Acquisitions and Dispositions, for additional information on the deconsolidation of the Russian businesses. The Company continues to monitor the ongoing situation and its potential impact on our business, financial condition, results of operations and cash flows. Business Update Related To COVID-19 For over two years, the COVID-19 pandemic has impacted businesses globally including in every geography in which the Company operates. Our businesses have remained resilient throughout the pandemic and demand for our advice and services remains strong. The Company had strong revenue growth the first three months of 2022. However, uncertainty remains in the economic outlook, and the ultimate extent of the impact of COVID-19 to the Company will depend on future developments that it is unable to predict, including new "waves" of infection from emerging variants of the virus and potential renewed restrictions and mandates by various governments or agencies.