Nature of Operations	3 Months Ended
Nature of Operations	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations Marsh & McLennan Companies, Inc. and its consolidated subsidiaries (the "Company") is a global professional services firm offering clients advice and solutions in risk, strategy and people. Its businesses include: Marsh, the insurance broker, intermediary and risk advisor; Guy Carpenter, the risk and reinsurance specialist; Mercer, the provider of HR and Investment related financial advice and services; and Oliver Wyman Group, the management, economic and brand consultancy. With 76,000 colleagues worldwide and annual revenue of $17 billion, the Company provides analysis, advice and transactional capabilities to clients in more than 130 countries. Business Update Related To COVID-19 In March 2020, the World Health Organization declared the Coronavirus (COVID-19) a pandemic. The outbreak has become increasingly widespread around the world, including virtually every geography in which the Company operates. The pandemic has created uncertainty about the impact on the global economy and has resulted in impacts to the financial markets and asset values. Governments have implemented various restrictions around the world, including closure of non-essential businesses, travel restrictions, shelter-in-place requirements for citizens and other restrictions. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition will depend on future developments that are uncertain and cannot be accurately predicted. This includes new information that may emerge concerning COVID-19, the actions taken to contain it or treat its impact and the economic impact on local, regional, national and international markets. The Company has taken a number of precautionary steps to safeguard its businesses and colleagues from COVID-19, including implementing travel restrictions, arranging work from home capabilities and flexible work policies. By the end of March, nearly all of the Company’s colleagues were working fully in a remote work environment, with virtually no disruption in the ability to serve clients. The safety and well-being of our colleagues is our first priority. The Company has established a $5 million support fund for colleagues in need, as well as matching gifts for charitable contributions. The Company is monitoring and assessing the impact of the COVID-19 pandemic on a daily basis to ensure that it continues to adhere to guidelines and orders issued by federal, state and local governments. The ultimate extent of the COVID-19 impact to the Company will depend on numerous evolving factors and future developments that it is not able to predict. Factors that could adversely affect the Company’s financial statements related to the financial and operational impact of COVID-19 are outlined in “Item 1A - Risk Factors” in the Company’s Form 10-Q. JLT Acquisition On April 1, 2019, the Company completed its previously announced acquisition (the "Transaction") of all of the outstanding shares of Jardine Lloyd Thompson Group plc ("JLT"), a public company organized under the laws of England and Wales. JLT's results of operations for the three month period ended March 31, 2020 are included in the Company's results of operations. The first quarter of 2019 does not reflect JLT’s results of operations for that period and therefore may affect comparability.