Segment Information (Segments and Geographic Areas) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	$ 26,981	$ 24,458	$ 22,736
Compensation and benefits	15,577	13,996	13,099
Depreciation and amortization expense	361	369	370
Identified intangible amortization expense	549	377	343
Other operating expenses	4,271	3,899	3,642
Operating income	6,223	5,817	5,282
Risk and Insurance Services
Segment Reporting, Asset Reconciling Item [Line Items]
Fiduciary interest income	403	497	453
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	27,059	24,528	22,798
Compensation and benefits	15,421	13,857	12,951
Depreciation and amortization expense	304	306	296
Identified intangible amortization expense	549	377	343
Other operating expenses	4,253	3,853	3,597
Operating income	6,532	6,135	5,611
Operating Segments \| Risk and Insurance Services
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	17,265	15,395	14,089
Compensation and benefits	9,711	8,499	7,702
Depreciation and amortization expense	204	192	190
Identified intangible amortization expense	475	326	297
Other operating expenses	2,239	2,013	1,955
Operating income	4,636	4,365	3,945
Fiduciary interest income	403	497	453
Gain related to investments in private equity funds and other investments	24	22	18
Operating Segments \| Risk and Insurance Services \| Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Pre-tax gain (loss) on sale	28
Operating Segments \| Consulting
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	9,794	9,133	8,709
Compensation and benefits	5,710	5,358	5,249
Depreciation and amortization expense	100	114	106
Identified intangible amortization expense	74	51	46
Other operating expenses	2,014	1,840	1,642
Operating income	1,896	1,770	1,666
Operating Segments \| Consulting \| Marsh Management Consulting
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	3,604	3,390	3,122
Pre-tax gain (loss) on sale		20
Operating Segments \| Consulting \| U.S. Health And Benefits And U.K Pension Administration Businesses
Segment Reporting, Asset Reconciling Item [Line Items]
Pre-tax gain (loss) on sale		35
Corporate/Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	(78)	(70)	(62)
Compensation and benefits	156	139	148
Depreciation and amortization expense	57	63	74
Identified intangible amortization expense	0	0	0
Other operating expenses	18	46	45
Operating income	(309)	(318)	(329)
Inter-segment \| Consulting
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	$ 73	$ 60	$ 56

MARSH & MCLENNAN COMPANIES, INC. 10-K