Nature Of Operations	9 Months Ended
Sep. 30, 2011
Nature Of Operations [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 organizational structure, the Company’s two business segments are Risk and Insurance Services and Consulting. The Risk and Insurance Services segment provides risk management 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. In January 2011, Marsh acquired RJF Agencies, an independent insurance agency in the upper Midwest region of the U.S. In February 2011, Marsh acquired Hampton Roads Bonding, a surety bonding agency for commercial, road, utility, maritime and government contractors in the state of Virginia, and the Boston office of Kinloch Consulting Group, Inc. In July 2011, Marsh acquired Prescott Pailet Benefits, an employee benefits broker in the state of Texas. In the first quarter of 2010, Marsh acquired Haake Companies, Inc., an insurance broking firm in the Midwest region and Thomas Rutherfoord, Inc., an insurance broking firm in the Southeast and mid-Atlantic regions of the U.S. In the second quarter of 2010, Marsh acquired HSBC Insurance Brokers Ltd., an international provider of risk intermediary and risk advisory services and the Bostonian Group Insurance Agency, Inc. and Bostonian Solutions, Inc. (collectively the “Bostonian Group”), a regional insurance brokerage in New England. In the fourth quarter of 2010 Marsh acquired Trion, a U.S. private benefits specialist and SBS, a Georgia-based benefits brokerage and consulting firm. The Consulting segment provides advice and services to the managements of organizations in the area of human resource consulting, comprising retirement and investments, health and benefits, outsourcing and talent; and strategy and risk management consulting, comprising management, economic and brand consulting. The Company conducts business in this segment through Mercer and Oliver Wyman Group. In the first quarter of 2011, Mercer acquired Hammond Associates, an investment consulting company for endowments and foundations in the U.S. In June 2011, Mercer acquired Evaluation Associates LLC, an investment consulting firm. In July 2011, Mercer acquired Mahoney Associates, a health and benefits advisory firm based in south Florida. In July 2010, Mercer acquired Innovative Process Administration (“IPA”), a provider of health and benefit recordkeeping and employee enrollment technology. In August 2010, Mercer acquired ORC Worldwide, a premier provider of HR knowledge, data and solutions for professionals in numerous industries. On August 3, 2010, the Company completed the sale of Kroll, the Company’s former Risk Consulting & Technology segment, to Altegrity, Inc. (“Altegrity”) for cash consideration of $1.13 billion. In the first quarter of 2010, Kroll completed the sale of Kroll Laboratory Specialists (“KLS”). The gain on the sale of Kroll and related tax benefits and the after-tax loss on the sale of KLS, along with Kroll’s, and KLS’s 2010 comparative results of operations are included in discontinued operations in 2010. With the sale of Kroll in August 2010, along with previous divestiture transactions between 2008 and 2010, the Company has divested its entire Risk Consulting & Technology segment. The run-off of the Company’s involvement in the Corporate Advisory and Restructuring business (“CARG”), previously part of Risk Consulting & Technology, in which the Company has “continuing involvement” as defined in SEC Staff Accounting Bulletin Topic 5e, is now managed by the Company’s corporate departments. Consequently, the financial results of the CARG businesses are included in “Corporate” for segment reporting purposes.

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 organizational structure, the Company’s two business segments are Risk and Insurance Services and Consulting.

The Risk and Insurance Services segment provides risk management 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.

In January 2011, Marsh acquired RJF Agencies, an independent insurance agency in the upper Midwest region of the U.S. In February 2011, Marsh acquired Hampton Roads Bonding, a surety bonding agency for commercial, road, utility, maritime and government contractors in the state of Virginia, and the Boston office of Kinloch Consulting Group, Inc. In July 2011, Marsh acquired Prescott Pailet Benefits, an employee benefits broker in the state of Texas.

In the first quarter of 2010, Marsh acquired Haake Companies, Inc., an insurance broking firm in the Midwest region and Thomas Rutherfoord, Inc., an insurance broking firm in the Southeast and mid-Atlantic regions of the U.S. In the second quarter of 2010, Marsh acquired HSBC Insurance Brokers Ltd., an international provider of risk intermediary and risk advisory services and the Bostonian Group Insurance Agency, Inc. and Bostonian Solutions, Inc. (collectively the “Bostonian Group”), a regional insurance brokerage in New England. In the fourth quarter of 2010 Marsh acquired Trion, a U.S. private benefits specialist and SBS, a Georgia-based benefits brokerage and consulting firm.

The Consulting segment provides advice and services to the managements of organizations in the area of human resource consulting, comprising retirement and investments, health and benefits, outsourcing and talent; and strategy and risk management consulting, comprising management, economic and brand consulting. The Company conducts business in this segment through Mercer and Oliver Wyman Group. In the first quarter of 2011, Mercer acquired Hammond Associates, an investment consulting company for endowments and foundations in the U.S. In June 2011, Mercer acquired Evaluation Associates LLC, an investment consulting firm. In July 2011, Mercer acquired Mahoney Associates, a health and benefits advisory firm based in south Florida. In July 2010, Mercer acquired Innovative Process Administration (“IPA”), a provider of health and benefit recordkeeping and employee enrollment technology. In August 2010, Mercer acquired ORC Worldwide, a premier provider of HR knowledge, data and solutions for professionals in numerous industries.

On August 3, 2010, the Company completed the sale of Kroll, the Company’s former Risk Consulting & Technology segment, to Altegrity, Inc. (“Altegrity”) for cash consideration of $1.13 billion. In the first quarter of 2010, Kroll completed the sale of Kroll Laboratory Specialists (“KLS”). The gain on the sale of Kroll and related tax benefits and the after-tax loss on the sale of KLS, along with Kroll’s, and KLS’s 2010 comparative results of operations are included in discontinued operations in 2010.

With the sale of Kroll in August 2010, along with previous divestiture transactions between 2008 and 2010, the Company has divested its entire Risk Consulting & Technology segment. The run-off of the Company’s involvement in the Corporate Advisory and Restructuring business (“CARG”), previously part of Risk Consulting & Technology, in which the Company has “continuing involvement” as defined in SEC Staff Accounting Bulletin Topic 5e, is now managed by the Company’s corporate departments. Consequently, the financial results of the CARG businesses are included in “Corporate” for segment reporting purposes.