Acquisitions and Dispositions (Narrative) (Details) $ in Millions	1 Months Ended	3 Months Ended					9 Months Ended		12 Months Ended
	Apr. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2023 USD ($) acquisition	Sep. 30, 2022 USD ($)	Dec. 31, 2022 acquisition	Nov. 01, 2022	Oct. 31, 2022
Business Acquisition [Line Items]
Total consideration							$ 711	$ 227
Cash paid							682	210
Estimated fair value of deferred/contingent consideration							29	17
Deferred purchase consideration from prior years' acquisitions							52	116
Contingent consideration from prior year's acquisitions							135	28
Cash and cash equivalents held in fiduciary capacity				$ 20			(18)	138
Deconsolidation of Russian businesses						$ (39)	0	(39)
Purchase of non-controlling interests			$ 139				139	0
Beassur SARL
Business Acquisition [Line Items]
Percentage of ownership in equity investment										70.00%	35.00%
U.S. Affinity Business
Business Acquisition [Line Items]
Cash proceeds from sale	$ 140
Gain on disposal	$ 112
Certain Businesses Primarily in Brazil
Business Acquisition [Line Items]
Cash proceeds from sale								7
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions								116
Contingent consideration from prior year's acquisitions							176	66
Pro forma revenue					$ 16			22
Operating income (loss) related to acquisitions					$ 1			$ 1
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration							711
Cash paid							682
Estimated fair value of deferred/contingent consideration							29
Pro forma revenue		$ 50					91
Operating income (loss) related to acquisitions		5					14
Advance Asset Management
Business Acquisition [Line Items]
Acquisition related costs		$ 5					32
Small Individual Financial Advisory Business | Operating Segments
Business Acquisition [Line Items]
Pre-tax loss on sale							$ 17
Risk and Insurance Services
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition							8		16
Consulting
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition							4		4
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