Acquisitions and Dispositions (Narrative) (Details) $ in Millions		3 Months Ended			12 Months Ended
	Jan. 01, 2024 USD ($)	Mar. 31, 2024 USD ($) acquisition	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 acquisition
Business Acquisition [Line Items]
Total consideration		$ 359		$ 15
Cash paid		308		11
Estimated fair value of deferred/contingent purchase consideration		51		4
Deferred purchase consideration from prior years' acquisitions		3		12
Contingent consideration from prior year's acquisitions		12		1
Cash divested from deconsolidation				20
Purchase of non-controlling interests			$ 139
Total Segments | Small Individual Financial Advisory Business
Business Acquisition [Line Items]
Pre-tax loss on sale				19
Net gain includes in revenue consolidated statements of income				(19)
Small Individual Financial Advisory Business | Total Segments | Discontinued Operations, Disposed of by Sale
Business Acquisition [Line Items]
Pre-tax loss on sale		(21)
Net gain includes in revenue consolidated statements of income		21
U.S. Health And Benefits And U.K Pension Administration Businesses | Discontinued Operations, Disposed of by Sale
Business Acquisition [Line Items]
Cash proceeds from sale	$ 114
Pre-tax loss on sale	(21)
Net gain includes in revenue consolidated statements of income	21
Exit costs	$ 18
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration		359
Cash paid		308
Estimated fair value of deferred/contingent purchase consideration		51
Pro forma revenue		14
Operating loss related to acquisitions		6
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions				12
Contingent consideration from prior year's acquisitions		26		1
Westpac Transaction
Business Acquisition [Line Items]
Acquisition related costs		$ 3		$ 17
Risk and Insurance Services
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		2			9
Consulting
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		4			5
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