Acquisitions and Dispositions (Narrative) (Details) $ in Millions		3 Months Ended		6 Months Ended	12 Months Ended
	Jan. 01, 2024 USD ($)	Mar. 31, 2025 USD ($) acquisition	Mar. 31, 2024 USD ($)	Jun. 30, 2024 acquisition	Dec. 31, 2024 acquisition	Jan. 31, 2025
Business Acquisition [Line Items]
Total consideration		$ 62	$ 359
Cash paid		37	308
Contingent purchase consideration		10	51
Fair value of previously-held equity method investment		15
Deferred purchase consideration from prior years' acquisitions		27	3
Contingent consideration from prior year's acquisitions		5	12
Acquisition related costs		78	$ 3
Marsh McLennan Agency's ("MMA") Technology Consulting And Administrative Solutions | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Business Acquisition [Line Items]
Cash proceeds from sale		25
Net gain includes 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 Acquisition [Line Items]
Cash proceeds from sale	$ 114
Exit costs	$ 18
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)		2 years	2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)		4 years	4 years
Carpenter Turner Cyprus Ltd.
Business Acquisition [Line Items]
Ownership share (percent)						51.50%
Gain on previously-held equity method investment, fair value		$ 13
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions			$ 3
Contingent consideration from prior year's acquisitions		19	$ 26
Mc Griff Insurance Services L L C
Business Acquisition [Line Items]
Acquisition related costs		$ 69
Risk and Insurance Services
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		3			10
Consulting
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition				7