Segment Information (Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenue	$ 4,416	$ 3,968	$ 4,189	$ 4,651	$ 4,264	$ 3,968	$ 4,349	$ 4,071	$ 17,224	$ 16,652	$ 14,950
Operating Income (Loss)	571	$ 540	$ 885	$ 1,070	592	$ 467	$ 680	$ 938	3,066	2,677	2,761
Total Assets	33,049				31,357				33,049	31,357	21,578
Depreciation and Amortization									741	647	494
Capital Expenditures									348	421	314
Equity method income										12	17
Equity method investments	280				434				280	434
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									10,337	9,599	8,228
Interest income, fiduciary assets									46	105	65
Consulting
Segment Reporting Information [Line Items]
Revenue									6,976	7,143	6,779
Interest income, fiduciary assets									1	4	3
Operating Segments
Segment Reporting Information [Line Items]
Revenue									17,313	16,742	15,007
Operating Income (Loss)									3,340	3,043	2,963
Total Assets	30,183				35,820				30,183	35,820	23,871
Depreciation and Amortization									674	572	420
Capital Expenditures									277	334	255
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									10,337	9,599	8,228
Operating Income (Loss)									2,346	1,833	1,864
Total Assets	20,612				26,098				20,612	26,098	15,868
Depreciation and Amortization									500	416	290
Capital Expenditures									170	184	158
Interest income, fiduciary assets									46	105	65
Equity method income									27	25	13
Proceeds from sale of business											40
Equity method investments	165				179				165	179	57
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									6,976	7,143	6,779
Operating Income (Loss)									994	1,210	1,099
Total Assets	9,571				9,722				9,571	9,722	8,003
Depreciation and Amortization									174	156	130
Capital Expenditures									107	150	97
Interest income, fiduciary assets									1	4	3
Equity method income									5	16	8
Equity method investments	5				149				5	149	148
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue									(89)	(90)	(57)
Operating Income (Loss)									(274)	(366)	(202)
Total Assets	$ 2,866				$ (4,463)				2,866	(4,463)	(2,293)
Depreciation and Amortization									67	75	74
Capital Expenditures									71	87	59
Inter-segment | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									5	8	6
Inter-segment | Consulting
Segment Reporting Information [Line Items]
Revenue									$ 84	$ 82	$ 51