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Nature of Operations	3 Months Ended
Nature of Operations	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations Marsh & McLennan Companies, Inc. and its consolidated subsidiaries (the "Company"), a global professional services firm, is organized based on the different services that it offers. Under this structure, the Company’s two segments are Risk and Insurance Services and Consulting. The Risk and Insurance Services segment provides risk management solutions, services, advice and insurance broking, reinsurance broking and insurance program management 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. The Company conducts business in its Consulting segment through Mercer and Oliver Wyman Group. Mercer provides consulting expertise, advice, services and solutions in the areas of health, wealth and career consulting services and products. Oliver Wyman Group provides specialized management and economic and brand consulting services. 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. Acquisitions and dispositions impacting the Risk and Insurance Services and Consulting segments are discussed in Note 8 to the consolidated financial statements.