Segment Information (Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenue		$ 3,364	$ 3,135	$ 3,376	$ 3,336	$ 3,338	$ 3,115	$ 3,225	$ 3,215	$ 13,211	$ 12,893	$ 12,951
Operating Income (Loss)		633	$ 572	$ 726	$ 733	594	$ 461	$ 629	$ 735	2,664	2,419	2,301
Total Assets		18,190				18,216				18,190	18,216	17,793
Depreciation and Amortization										438	423	388
Capital Expenditures										253	325	368
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest income, fiduciary assets										26	21	24
Consulting Segment
Segment Reporting Information [Line Items]
Interest income, fiduciary assets										3	4	6
Operating Segments
Segment Reporting Information [Line Items]
Revenue										13,255	12,933	12,990
Operating Income (Loss)										2,856	2,614	2,505
Total Assets		21,498				19,775				21,498	19,775	18,127
Depreciation and Amortization										369	360	332
Capital Expenditures										196	244	265
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]									7,143	6,869	6,931
Operating Income (Loss)										1,753	1,539	1,509
Total Assets		14,728				13,290				14,728	13,290	12,211
Depreciation and Amortization										248	240	213
Capital Expenditures										128	136	173
Interest income, fiduciary assets										26	21	24
Equity method income										12	6	9
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]									6,112	6,064	6,059
Operating Income (Loss)										1,103	1,075	996
Total Assets		6,770				6,485				6,770	6,485	5,916
Depreciation and Amortization										121	120	119
Capital Expenditures										68	108	92
Interest income, fiduciary assets										3	4	6
Equity method income										19	21	2
Corporate and Elimination
Segment Reporting Information [Line Items]
Revenue										(44)	(40)	(39)
Operating Income (Loss)										(192)	(195)	(204)
Total Assets	[3]	$ (3,308)				$ (1,559)				(3,308)	(1,559)	(334)
Depreciation and Amortization										69	63	56
Capital Expenditures										57	81	103
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										6	6	4
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										$ 38	$ 34	$ 35

[1] Includes inter-segment revenue of $6 million in both 2016 and 2015 and $4 million in 2014, interest income on fiduciary funds of $26 million, $21 million and $24 million in 2016, 2015 and 2014, respectively, and equity method income of $12 million, $6 million and $9 million in 2016, 2015 and 2014, respectively.

[2] Includes inter-segment revenue of $38 million, $34 million and $35 million in 2016, 2015 and 2014, respectively, interest income on fiduciary funds of $3 million in 2016, $4 million in 2015 and $6 million in 2014 and equity method income of $19 million in 2016, $21 million in 2015 and $2 million in 2014.

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