Segment Information (Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenue		$ 3,338	$ 3,115	$ 3,225	$ 3,215	$ 3,246	$ 3,141	$ 3,300	$ 3,264	$ 12,893	$ 12,951	$ 12,261
Operating Income (Loss)		594	$ 461	$ 629	$ 735	536	$ 445	$ 647	$ 673	2,419	2,301	2,077
Total Assets		18,216				17,793				18,216	17,793	16,960
Depreciation and Amortization										423	388	358
Capital Expenditures										325	368	401
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest income, fiduciary assets										21	24	27
Consulting Segment
Segment Reporting Information [Line Items]
Interest income, fiduciary assets										4	6	5
Operating Segments
Segment Reporting Information [Line Items]
Revenue										12,933	12,990	12,297
Operating Income (Loss)										2,614	2,505	2,266
Total Assets		19,775				18,127				19,775	18,127	16,543
Depreciation and Amortization										360	332	307
Capital Expenditures										244	265	313
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]									6,869	6,931	6,596
Operating Income (Loss)										1,539	1,509	1,421
Total Assets		13,290				12,211				13,290	12,211	11,365
Depreciation and Amortization										240	213	192
Capital Expenditures										136	173	158
Interest income, fiduciary assets										21	24	27
Equity method income										6	9	8
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]									6,064	6,059	5,701
Operating Income (Loss)										1,075	996	845
Total Assets		6,485				5,916				6,485	5,916	5,178
Depreciation and Amortization										120	119	115
Capital Expenditures										108	92	155
Interest income, fiduciary assets										4	6	5
Equity method income										21	2	0
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue										(40)	(39)	(36)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										6	4	5
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										34	35	31
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenue										(40)	(39)	(36)
Operating Income (Loss)										(195)	(204)	(189)
Total Assets	[3]	$ (1,559)				$ (334)				(1,559)	(334)	417
Depreciation and Amortization										63	56	51
Capital Expenditures										$ 81	$ 103	$ 88

[1] Includes inter-segment revenue of $6 million in 2015, $4 million in 2014 and $5 million in 2013, interest income on fiduciary funds of $21 million, $24 million and $27 million in 2015, 2014 and 2013, respectively, and equity method income of $6 million, $9 million and $8 million in 2015, 2014 and 2013, respectively.

[2] Includes inter-segment revenue of $34 million, $35 million and $31 million in 2015, 2014 and 2013, respectively, interest income on fiduciary funds of $4 million in 2015, $6 million in 2014 and $5 million in 2013 and equity method income of $21 million in 2015, $2 million in 2014 and $0 million in 2013.

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