Acquisitions and Dispositions (Narrative) (Details) $ in Millions		1 Months Ended		3 Months Ended			12 Months Ended
	Jan. 01, 2024 USD ($)	Apr. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2023 USD ($) acquisition	Dec. 31, 2022 USD ($) acquisition	Dec. 31, 2021 USD ($)	Nov. 01, 2022	Oct. 31, 2022
Business Acquisition [Line Items]
Total purchase consideration							$ 1,200	$ 705
Cash paid							1,100	579
Estimated contingent consideration							41	120	$ 153
Fair value of previously-held equity method investment							0	6	390
Deferred purchase consideration							202	158	117
Revenue related to acquisitions							152	58	114
Net operating income (loss) related to acquisitions							24	(5)	3
Acquisition-related deposit							45	29
Cash and cash equivalents held in fiduciary capacity					$ 24		(17)	119	84
Cash and cash equivalents held in a fiduciary capacity								36
Deconsolidation of Russian businesses						$ 39	0	39	0
Payments to Noncontrolling Interests				$ 139			139	$ 7	0
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration]								Revenue
Marsh India
Business Acquisition [Line Items]
Step acquisition, remeasurement gain			$ 267
U.S. Affinity Business
Business Acquisition [Line Items]
Cash proceeds from sale		$ 140
Net gain on disposal		$ 112
Certain Businesses Primarily in the U.S., U.K. and Canada
Business Acquisition [Line Items]
Cash proceeds from sale								$ 15
Gain on divestiture of businesses								$ 15
U.S. Health And Benefits And U.K Pension Administration Businesses | Discontinued Operations, Held-for-Sale
Business Acquisition [Line Items]
Net assets as assets held for sale							$ 70
U.S. Health And Benefits And U.K Pension Administration Businesses | Subsequent Event | Sale
Business Acquisition [Line Items]
Cash proceeds from sale	$ 110
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
Westpac Transaction
Business Acquisition [Line Items]
Acquisition-related deposit							$ 39	$ 21
Beassur SARL
Business Acquisition [Line Items]
Percentage of voting interests acquired										70.00%	35.00%
Acquisitions Made In Prior Years
Business Acquisition [Line Items]
Estimated contingent consideration							67	126
Deferred purchase consideration							$ 176	$ 70
Risk and Insurance Services
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition							9	16
Risk and Insurance Services | Operating Segments
Business Acquisition [Line Items]
Loss on sale of business									$ 50
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition							5	4
Small Individual Financial Advisory Business | Operating Segments
Business Acquisition [Line Items]
Loss on sale of business							$ (17)