Acquisitions and Dispositions (Narrative) (Details) $ in Millions		3 Months Ended				9 Months Ended		12 Months Ended
	Jan. 01, 2024 USD ($)	Sep. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Sep. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2025 USD ($) acquisition	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($) acquisition	Jan. 31, 2025
Business Combination [Line Items]
Total consideration						$ 337	$ 1,300
Cash paid						254	1,100
Contingent purchase consideration						68	176
Fair value of previously-held equity method investment						15
Deferred purchase consideration from prior years' acquisitions						53	18
Contingent consideration from prior year's acquisitions						11	73
Acquisition related costs		$ 81		$ 20		$ 218	$ 49
Marsh McLennan Agency's ("MMA") Technology Consulting And Administrative Solutions | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Business Combination [Line Items]
Cash proceeds from sale			$ 25
Net gain included in revenue consolidated statements of income			15
U.S. Health And Benefits And U.K Pension Administration Businesses | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Business Combination [Line Items]
Cash proceeds from sale	$ 114
Exit costs					$ 18
Certain Businesses Primarily in Brazil
Business Combination [Line Items]
Cash proceeds from sale								$ 21
Minimum
Business Combination [Line Items]
Revenue target period (in years)						2 years	2 years
Maximum
Business Combination [Line Items]
Revenue target period (in years)						4 years	4 years
Carpenter Turner Cyprus Ltd.
Business Combination [Line Items]
Ownership share (percent)									51.50%
Gain on previously-held equity method investment, fair value			$ 13
Prior Fiscal Periods Acquisitions
Business Combination [Line Items]
Deferred purchase consideration from prior years' acquisitions							$ 18
Contingent consideration from prior year's acquisitions						$ 38	$ 163
Mc Griff Insurance Services L L C
Business Combination [Line Items]
Acquisition related costs		$ 52				$ 166
Risk and Insurance Services
Business Combination [Line Items]
Number of acquisitions made (in acquisitions) | acquisition						8		10
Consulting
Business Combination [Line Items]
Number of acquisitions made (in acquisitions) | acquisition						4		7