Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenue	$ 7,061	$ 6,473
Compensation and benefits	3,850	3,470
Depreciation and amortization expense	88	99
Amortization of intangible assets	139	90
Other operating expenses	979	889
Operating income	2,005	1,925
Operating Segments
Segment Reporting Information [Line Items]
Revenue	7,076	6,487
Compensation and benefits	3,814	3,432
Depreciation and amortization expense	74	83
Amortization of intangible assets	139	90
Other operating expenses	980	885
Operating income	2,069	1,997
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	4,762	4,273
Compensation and benefits	2,451	2,118
Depreciation and amortization expense	50	46
Amortization of intangible assets	120	79
Other operating expenses	528	465
Operating income	1,613	1,565
Interest on fiduciary funds	103	122
Operating Segments | Risk and Insurance Services | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Net gain includes in revenue consolidated statements of income	(28)
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	2,314	2,214
Compensation and benefits	1,363	1,314
Depreciation and amortization expense	24	37
Amortization of intangible assets	19	11
Other operating expenses	452	420
Operating income	456	432
Operating Segments | Consulting | U.S. Health And Benefits And U.K Pension Administration Businesses
Segment Reporting Information [Line Items]
Net gain includes in revenue consolidated statements of income			$ (21)
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(15)	(14)
Compensation and benefits	36	38
Depreciation and amortization expense	14	16
Amortization of intangible assets	0	0
Other operating expenses	(1)	4
Operating income	(64)	(72)
Intersegment Eliminations | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 15	$ 13