Segment Information (Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$ 3,246		$ 3,141	$ 3,300	$ 3,264	$ 3,115		$ 2,932	$ 3,088	$ 3,126	$ 12,951		$ 12,261		$ 11,924
Operating Income (Loss)	536		445	647	673	489		404	577	607	2,301		2,077		1,829
Total Assets	17,840					16,980					17,840		16,980		16,288
Depreciation and Amortization											388		358		349
Capital Expenditures											368		401		320
Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Interest Income, Fiduciary Assets											24		27		39
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Interest Income, Fiduciary Assets											6		5		4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue											12,990		12,297		11,963
Operating Income (Loss)											2,505		2,266		2,026
Total Assets	18,127					16,543					18,127		16,543		15,035
Depreciation and Amortization											332		307		309
Capital Expenditures											265		313		248
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue											6,931	[1]	6,596	[1]	6,350	[1]
Operating Income (Loss)											1,509		1,421		1,334
Total Assets	12,211					11,365					12,211		11,365		9,832
Depreciation and Amortization											213		192		196
Capital Expenditures											173		158		131
Interest Income, Fiduciary Assets											24		27		39
Equity method income											9		8		11
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue											6,059	[2]	5,701	[2]	5,613	[2]
Operating Income (Loss)											996		845		692
Total Assets	5,916					5,178					5,916		5,178		5,203
Depreciation and Amortization											119		115		113
Capital Expenditures											92		155		117
Interest Income, Fiduciary Assets											6		5		4
Equity method income											2		0		0
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue											(39)		(36)		(39)
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue											4		5		5
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue											35		31		34
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)											(204)		(189)		(197)
Total Assets	(287)	[3]				437	[3]				(287)	[3]	437	[3]	1,253	[3]
Depreciation and Amortization											56		51		40
Capital Expenditures											$ 103		$ 88		$ 72

[1] Includes inter-segment revenue of $4 million in 2014 and $5 million in both 2013 and 2012, interest income on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively, and equity method income of $9 million, $8 million and $11 million in 2014, 2013 and 2012, respectively.

[2] Includes inter-segment revenue of $35 million, $31 million and $34 million in 2014, 2013 and 2012, respectively, interest income on fiduciary funds of $6 million in 2014, $5 million in 2013 and $4 million 2012 and equity method income of $2 million in 2014, and $0 million in both 2013 and 2012.

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