Segment Information (Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenue	$ 4,264	$ 3,968	$ 4,349	$ 4,071	$ 3,712	$ 3,504	$ 3,734	$ 4,000	$ 16,652	$ 14,950	$ 14,024
Operating Income (Loss)	592	$ 467	$ 680	$ 938	621	$ 541	$ 691	$ 908	2,677	2,761	2,655
Total Assets	31,357				21,578				31,357	21,578	20,429
Depreciation and Amortization									647	494	481
Capital Expenditures									421	314	302
Equity method income									12	17
Equity method investments	434				287				434	287
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									9,599	8,228	7,630
Interest income, fiduciary assets									105	65	39
Consulting
Segment Reporting Information [Line Items]
Revenue									7,143	6,779	6,444
Interest income, fiduciary assets									4	3	4
Operating Segments
Segment Reporting Information [Line Items]
Revenue									16,742	15,007	14,074
Operating Income (Loss)									3,043	2,963	2,841
Total Assets	35,820				23,871				35,820	23,871	24,690
Depreciation and Amortization									572	420	411
Capital Expenditures									334	255	227
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									9,599	8,228	7,630
Operating Income (Loss)									1,833	1,864	1,731
Total Assets	26,098				15,868				26,098	15,868	16,490
Depreciation and Amortization									416	290	282
Capital Expenditures									184	158	139
Interest income, fiduciary assets									105	65	39
Equity method income									25	13	14
Proceeds from sale of business										40
Equity method investments	179				57				179	57	60
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									7,143	6,779	6,444
Operating Income (Loss)									1,210	1,099	1,110
Total Assets	9,722				8,003				9,722	8,003	8,200
Depreciation and Amortization									156	130	129
Capital Expenditures									150	97	88
Interest income, fiduciary assets									4	3	4
Equity method income									16	8	17
Equity method investments	149				148				149	148	269
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue									(90)	(57)	(50)
Operating Income (Loss)									(366)	(202)	(186)
Total Assets	$ (4,463)				$ (2,293)				(4,463)	(2,293)	(4,261)
Depreciation and Amortization									75	74	70
Capital Expenditures									87	59	75
Inter-segment | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									8	6	5
Inter-segment | Consulting
Segment Reporting Information [Line Items]
Revenue									$ 82	$ 51	$ 45