Segment Information (Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue		$ 3,712	$ 3,504	$ 3,734	$ 4,000	$ 3,685	$ 3,341	$ 3,495	$ 3,503	$ 14,950	$ 14,024	$ 13,211
Operating income		621	$ 541	$ 691	$ 908	670	$ 535	$ 701	$ 749	2,761	2,655	2,431
Total Assets		21,578				20,429				21,578	20,429	18,190
Depreciation and Amortization										494	481	438
Capital Expenditures										314	302	253
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest income, fiduciary assets										65	39	26
Consulting Segment
Segment Reporting Information [Line Items]
Interest income, fiduciary assets										3	4	3
Operating Segments
Segment Reporting Information [Line Items]
Revenue										15,007	14,074	13,255
Operating income										2,963	2,841	2,619
Total Assets		23,871				24,690				23,871	24,690	21,498
Depreciation and Amortization										420	411	369
Capital Expenditures										255	227	196
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]									8,228	7,630	7,143
Operating income										1,864	1,731	1,581
Total Assets		15,868				16,490				15,868	16,490	14,728
Depreciation and Amortization										290	282	248
Capital Expenditures										158	139	128
Interest income, fiduciary assets										65	39	26
Equity method income										13	14	12
Proceeds from sale of business										40
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]									6,779	6,444	6,112
Operating income										1,099	1,110	1,038
Total Assets		8,003				8,200				8,003	8,200	6,770
Depreciation and Amortization										130	129	121
Capital Expenditures										97	88	68
Interest income, fiduciary assets										3	4	3
Equity method income										8	17	19
Corporate and Elimination
Segment Reporting Information [Line Items]
Revenue										(57)	(50)	(44)
Operating income										(202)	(186)	(188)
Total Assets	[3]	$ (2,293)				$ (4,261)				(2,293)	(4,261)	(3,308)
Depreciation and Amortization										74	70	69
Capital Expenditures										59	75	57
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										6	5	6
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										$ 51	$ 45	$ 38

[1] Includes inter-segment revenue of $6 million, $5 million and $6 million in 2018, 2017 and 2016, respectively, interest income on fiduciary funds of $65 million, $39 million and $26 million in 2018, 2017 and 2016, respectively, and equity method income of $13 million, $14 million and $12 million in 2018, 2017 and 2016, respectively and $40 million related to the sale of business in 2018.

[2] Includes inter-segment revenue of $51 million, $45 million and $38 million in 2018, 2017 and 2016, respectively, interest income on fiduciary funds of $3 million, $4 million and $3 million in 2018, 2017 and 2016, respectively, and equity method income of $8 million, $17 million and $19 million in 2018, 2017 and 2016, respectively.

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