Nature of Operations	6 Months Ended
Nature of Operations	Jun. 30, 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 spread quickly around the world, including essentially every geography in which the Company operates. The pandemic has created uncertainty around the impact on the global economy and has resulted in impacts to the financial markets and asset values. Governments implemented various restrictions around the world, including closure of non-essential businesses, travel, shelter-in-place requirements for citizens and other restrictions. The Company took 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. Through the end of the second quarter, nearly all of the Company’s colleagues have continued working fully in a remote work environment, with virtually no disruption to the Company as a whole and its ability to serve clients. The safety and well-being of our colleagues continues to be our first priority. Over the past several months, many countries have begun to “re-open” their economies, with various conditions imposed by each jurisdiction. The Company has re-opened offices in various locations around the world, while ensuring that it continues to adhere to guidelines and orders issued by national, state and local governments. The timing of office re-openings will vary based on the conditions and restrictions in each location, but it remains uncertain the extent to which colleagues will return to offices for the remainder of the year. However, the Company expects it will continue its ability to service clients effectively while colleagues remain in a remote work environment. 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 for the quarter ended March 31, 2020. JLT Acquisition On April 1, 2019, the Company completed its 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 and six month periods ended June 30, 2020 are included in the Company's results of operations. The Company's results for the six months ended June 30, 2019 do not reflect JLT’s results of operations for the three months ended March 31, 2019 and therefore may affect comparability.