Segment Information (Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Revenue	$ 12,261		$ 11,924		$ 11,526
Operating Income (Loss)	2,077		1,829		1,638
Total Assets	16,980		16,288		15,454
Depreciation and Amortization	358		349		332
Capital Expenditures	401		320		280
Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Interest Income, Fiduciary Assets	27		39		47
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Interest Income, Fiduciary Assets	5		4		4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	12,297		11,963		11,566
Operating Income (Loss)	2,266		2,026		1,817
Total Assets	16,543		15,035		13,922
Depreciation and Amortization	307		309		301
Capital Expenditures	313		248		237
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	6,596	[1]	6,350	[1]	6,079	[1]
Operating Income (Loss)	1,421		1,334		1,200
Total Assets	11,365		9,832		9,102
Depreciation and Amortization	192		196		189
Capital Expenditures	158		131		146
Interest Income, Fiduciary Assets	27		39		47
Equity method income	8		11		14
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	5,701	[2]	5,613	[2]	5,487	[2]
Operating Income (Loss)	845		692		617
Total Assets	5,178		5,203		4,820
Depreciation and Amortization	115		113		112
Capital Expenditures	155		117		91
Interest Income, Fiduciary Assets	5		4		4
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(36)	[3]	(39)	[3]	(40)	[3]
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	5		5		4
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	31		34		36
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	(189)	[3]	(197)	[3]	(179)	[3]
Total Assets	437	[4]	1,253	[4]	1,532	[4]
Depreciation and Amortization	51		40		31
Capital Expenditures	$ 88		$ 72		$ 43

[1] Includes inter-segment revenue of $5 million in both 2013 and 2012 and $4 million in 2011, interest income on fiduciary funds of $27 million, $39 million and $47 million in 2013, 2012 and 2011, respectively, and equity method income of $8 million, $11 million and $14 million in 2013, 2012 and 2011, respectively.

[2] Includes inter-segment revenue of $31 million, $34 million and $36 million in 2013, 2012 and 2011, respectively, and interest income on fiduciary funds of $5 million in 2013, and $4 million in both 2012 and 2011.

[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.