Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Financial instruments owned:
Exchange traded equity securities	$ 19	$ 16
Fair Value, Measurements, Recurring
Financial instruments owned:
Total assets measured at fair value	544	790
Fiduciary Assets:
Total fiduciary assets measured at fair value	65	180
Liabilities:
Total liabilities measured at fair value	121	252
Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Total assets measured at fair value	544	789
Fiduciary Assets:
Total fiduciary assets measured at fair value	65	180
Liabilities:
Total liabilities measured at fair value	0	0
Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Total assets measured at fair value	0	0
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	0	0
Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Total assets measured at fair value	0	1
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	121	252
Money market funds | Fair Value, Measurements, Recurring
Fiduciary Assets:
Total fiduciary assets measured at fair value	65	180
Money market funds | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Total fiduciary assets measured at fair value	65	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
Other Assets | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	8	5
Mutual funds	182	178
Other Assets | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	8	5
Mutual funds	182	178
Other Assets | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Other Assets | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Money market funds | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	354	606
Money market funds | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	354	606
Money market funds | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	0	0
Money market funds | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	0	0
Contingent purchase consideration assets | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration assets	0	1
Contingent purchase consideration assets | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration assets	0	0
Contingent purchase consideration assets | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration assets	0	0
Contingent purchase consideration assets | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration assets	0	1
Contingent purchase consideration liabilities | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liabilities	121	252
Contingent purchase consideration liabilities | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liabilities	0	0
Contingent purchase consideration liabilities | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liabilities	0	0
Contingent purchase consideration liabilities | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liabilities	$ 121	$ 252