Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Financial instruments owned:
Exchange traded equity securities	$ 23	$ 19
Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	11	7
Mutual funds	204	194
Unit investment trust	0	83
Money market funds	135	353
Total assets measured at fair value	350	637
Fiduciary Assets:
Total fiduciary assets measured at fair value	215	76
Liabilities:
Contingent purchase consideration liabilities	230	161
Total liabilities measured at fair value	230	161
Fair Value, Measurements, Recurring | Money market funds
Fiduciary Assets:
Total fiduciary assets measured at fair value	215	76
Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	11	7
Mutual funds	204	194
Unit investment trust	0	0
Money market funds	135	353
Total assets measured at fair value	350	554
Fiduciary Assets:
Total fiduciary assets measured at fair value	215	76
Liabilities:
Contingent purchase consideration liabilities	0	0
Total liabilities measured at fair value	0	0
Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Money market funds
Fiduciary Assets:
Total fiduciary assets measured at fair value	215	76
Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Unit investment trust	0	83
Money market funds	0	0
Total assets measured at fair value	0	83
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
Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Money market funds
Fiduciary Assets:
Total fiduciary assets 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
Unit investment trust	0	0
Money market funds	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	230	161
Total liabilities measured at fair value	230	161
Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Money market funds
Fiduciary Assets:
Total fiduciary assets measured at fair value	$ 0	$ 0