Segment Information (Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenue	$ 11,924		$ 11,526		$ 10,550
Operating Income (Loss)	1,829		1,638		939
Total Assets	16,288		15,454
Risk and Insurance Services [Member]
Segment Reporting Information [Line Items]
Revenue	6,581	[1]	6,301	[1]	5,764	[1]
Operating Income (Loss)	1,374		1,229		972
Total Assets	9,932		9,202		9,418
Depreciation and Amortization	198		191		177
Capital Expenditures	131		146		144
Inter-segment revenue	5		4		7
Interest income on fiduciary funds	39		47		45
Equity method income	11		14		12
Consulting [Member]
Segment Reporting Information [Line Items]
Revenue	5,382	[2]	5,265	[2]	4,835	[2]
Operating Income (Loss)	652		588		129
Total Assets	5,103		4,720		4,437
Depreciation and Amortization	111		110		113
Capital Expenditures	117		91		80
Inter-segment revenue	34		36		43
Interest income on fiduciary funds	4		4		4
Total Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	11,963		11,566		10,599
Operating Income (Loss)	2,026		1,817		1,101
Total Assets	15,035		13,922		13,855
Depreciation and Amortization	309		301		290
Capital Expenditures	248		237		224
Corporate / Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(39)	[3]	(40)	[3]	(49)	[3]
Operating Income (Loss)	(197)	[3]	(179)	[3]	(162)	[3]
Total Assets	1,253	[4]	1,532	[4]	1,455	[4]
Depreciation and Amortization	40		31		29
Capital Expenditures	72		43		34
Total Consolidated [Member]
Segment Reporting Information [Line Items]
Revenue	11,924		11,526		10,550
Operating Income (Loss)	1,829		1,638		939
Total Assets	16,288		15,454		15,310
Depreciation and Amortization	349		332		319
Capital Expenditures	$ 320		$ 280		$ 258

[1] Includes inter-segment revenue of $5 million, $4 million and $7 million in 2012, 2011 and 2010, respectively, interest income on fiduciary funds of $39 million, $47 million and $45 million in 2012, 2011 and 2010, respectively, and equity method income of $11 million, $14 million and $12 million in 2012, 2011 and 2010, respectively.

[2] Includes inter-segment revenue of $34 million, $36 million and $43 million in 2012, 2011 and 2010, respectively, and interest income on fiduciary funds of $4 million in 2012, 2011 and 2010.

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

[4] Corporate assets primarily include insurance recoverables, pension related assets, the owned portion of the Company headquarters building and intercompany eliminations.