Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Financial instruments owned:
Exchange traded equity securities	$ 19	$ 16
Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	8	5
Mutual funds	189	178
Money market funds	440	606
Contingent purchase consideration assets	0	1
Total assets measured at fair value	637	790
Fiduciary Assets:
Total fiduciary assets measured at fair value	66	180
Liabilities:
Contingent purchase consideration liabilities	168	252
Total liabilities measured at fair value	168	252
Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	8	5
Mutual funds	189	178
Money market funds	440	606
Contingent purchase consideration assets	0	0
Total assets measured at fair value	637	789
Fiduciary Assets:
Total fiduciary assets measured at fair value	66	180
Liabilities:
Contingent purchase consideration liabilities	0	0
Total liabilities measured at fair value	0	0
Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Money market funds	0	0
Contingent purchase consideration assets	0	0
Total assets measured at fair value	0	0
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Contingent purchase consideration liabilities	0	0
Total liabilities measured at fair value	0	0
Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Money market funds	0	0
Contingent purchase consideration assets	0	1
Total assets measured at fair value	0	1
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Contingent purchase consideration liabilities	168	252
Total liabilities measured at fair value	168	252
Money market funds | Fair Value, Measurements, Recurring
Fiduciary Assets:
Total fiduciary assets measured at fair value	66	180
Money market funds | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Total fiduciary assets measured at fair value	66	180
Money market funds | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Money market funds | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Total fiduciary assets measured at fair value	$ 0	$ 0