Acquisitions and Dispositions (Narrative) (Details) $ in Millions			3 Months Ended			12 Months Ended
	Dec. 31, 2024 USD ($)	Jan. 01, 2024 USD ($)	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2024 USD ($) acquisition	Dec. 31, 2023 USD ($) acquisition	Dec. 31, 2022 USD ($)
Business Acquisition [Line Items]
Business combination, consideration transferred						$ 9,400	$ 1,200
Cash paid						9,200
Estimated fair value of deferred/contingent consideration						190
Deferred purchase consideration						113	202	$ 158
Acquisition-related deposit						119	45
Dispositions					$ 24	89	(17)	119
Payments to noncontrolling interests				$ 139		7	$ 139	$ 7
U.S. Health And Benefits And U.K Pension Administration Businesses
Business Acquisition [Line Items]
Pre-tax gain (loss) on sale						$ 35
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Oliver Wyman Group's Celent Advisory Business
Business Acquisition [Line Items]
Proceeds from sale of business	$ 24
Pre-tax gain (loss) on sale	$ 20
Discontinued Operations, Held-for-Sale | U.S. Health And Benefits And U.K Pension Administration Businesses
Business Acquisition [Line Items]
Proceeds from sale of business		$ 120
Pre-tax gain (loss) on sale		$ 35
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
Risk and Insurance Services
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition						10	9
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition						7	5
Consulting Segment | Operating Segments | Small Individual Financial Advisory Business
Business Acquisition [Line Items]
Pre-tax gain (loss) on sale						$ 35	$ 17
Mc Griff Insurance Services L L C
Business Acquisition [Line Items]
Acquisition-related deposit						63
Westpac Transaction
Business Acquisition [Line Items]
Acquisition-related deposit							39
Acquisitions Made In 2023
Business Acquisition [Line Items]
Cash paid							1,100
Estimated fair value of deferred/contingent consideration							41
Acquisitions Made In Prior Years
Business Acquisition [Line Items]
Estimated fair value of deferred/contingent consideration						39	67
Deferred purchase consideration						$ 166	$ 176