Segment Information (Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Segment Reporting Information [Line Items]
Revenue	$ 11,526		$ 10,550		$ 9,831
Operating Income (Loss)	1,638		939		778
Total Assets	15,454		15,310
Assets Of Discontined Operations And Corporate Advisory And Restructuring Business					1,600
Risk and Insurance Services [Member]
Segment Reporting Information [Line Items]
Revenue	6,301	[1]	5,764	[1]	5,284	[2]
Operating Income (Loss)	1,229		972		796
Total Assets	9,202		9,418		8,320
Depreciation and Amortization	191		177		153
Capital Expenditures	146		144		147
Inter-segment revenue	4		7		18
Interest income on fiduciary funds	47		45		54
Equity method income	14		12		13
Consulting [Member]
Segment Reporting Information [Line Items]
Revenue	5,265	[3]	4,835	[3]	4,609	[4]
Operating Income (Loss)	588		129		405
Total Assets	4,720		4,437		4,244
Depreciation and Amortization	110		113		114
Capital Expenditures	91		80		78
Inter-segment revenue	36		43		45
Interest income on fiduciary funds	4		4		4
Total Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	11,566		10,599		9,893
Operating Income (Loss)	1,817		1,101		1,201
Total Assets	13,922		13,855		12,564
Depreciation and Amortization	301		290		267
Capital Expenditures	237		224		225
Corporate / Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(40)	[2]	(49)	[2]	(62)	[2]
Operating Income (Loss)	(179)	[2]	(162)	[2]	(423)	[2]
Total Assets	1,532	[4]	1,455	[4]	2,773	[4]
Depreciation and Amortization	31		29		28
Capital Expenditures	43		34		36
Total Consolidated [Member]
Segment Reporting Information [Line Items]
Revenue	11,526		10,550		9,831
Operating Income (Loss)	1,638		939		778
Total Assets	15,454		15,310		15,337
Depreciation and Amortization	332		319		295
Capital Expenditures	$ 280		$ 258		$ 261

[1] Includes inter-segment revenue of $4 million, $7 million and $18 million in 2011, 2010 and 2009 respectively, interest income on fiduciary funds of $47 million, $45 million and $54 million in 2011, 2010, and 2009, respectively, and equity method income of $14 million, $12 million and $13 million in 2011, 2010, and 2009, respectively.

[2] Includes results of corporate advisory and restructuring business.

[3] Includes inter-segment revenue of $36 million, $43 million and $45 million in 2011, 2010 and 2009, respectively, and interest income on fiduciary funds of $4 million in 2011, 2010, and 2009.

[4] Corporate assets primarily include insurance recoverables, pension related assets, the owned portion of the Company headquarters building and intercompany eliminations. Also includes assets of discontinued operations and the corporate advisory and restructuring business of $1.6 billion in 2009.