Acquisitions and Dispositions (Narrative) (Details) $ in Millions			3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2024 USD ($)	Jan. 01, 2024 USD ($)	Sep. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) acquisition	Sep. 30, 2023 USD ($)	Dec. 31, 2023 acquisition	Dec. 31, 2024 USD ($)
Business Acquisition [Line Items]
Total consideration								$ 1,300	$ 711
Cash paid								1,100	682
Estimated fair value of deferred/contingent purchase consideration								176	29
Deferred purchase consideration from prior years' acquisitions								18	52
Contingent consideration from prior year's acquisitions								73	135
Cash divested from deconsolidation							$ 20
Purchase of non-controlling interests						$ 139		5	139
Total Segments | Small Individual Financial Advisory Business
Business Acquisition [Line Items]
Net gain (loss) on disposition of business									(17)
Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses
Business Acquisition [Line Items]
Cash proceeds from sale		$ 114
Net gain (loss) on disposition of business		$ 21
Exit costs				$ 18
Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses | Total Segments
Business Acquisition [Line Items]
Net gain (loss) on disposition of business								21
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions									52
Contingent consideration from prior year's acquisitions								163	176
Pro forma revenue					$ 50				91
Operating income (loss) related to acquisitions					5				14
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration								1,254
Cash paid								1,078
Estimated fair value of deferred/contingent purchase consideration								176
Pro forma revenue			$ 82					144
Operating income (loss) related to acquisitions			(2)					(5)
Westpac Transaction
Business Acquisition [Line Items]
Acquisition related costs			$ 20		$ 5			$ 49	$ 32
McGriff Insurance Services, LLC
Business Acquisition [Line Items]
Price of acquisition, expected	$ 7,750
McGriff Insurance Services, LLC | Forecast
Business Acquisition [Line Items]
Deferred tax assets											$ 500
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								8		9
Consulting
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition								5		5