Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Financial instruments owned:
Exchange traded equity securities	$ 82	$ 75
Fair Value, Measurements, Recurring
Financial instruments owned:
Total assets measured at fair value	340	691
Fiduciary Assets:
Fiduciary assets	235	582
Liabilities:
Total liabilities measured at fair value	358	352
Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Total assets measured at fair value	330	678
Fiduciary Assets:
Fiduciary assets	235	582
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	8	8
Fiduciary Assets:
Fiduciary assets	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	2	5
Fiduciary Assets:
Fiduciary assets	0	0
Liabilities:
Total liabilities measured at fair value	358	352
U.S. treasury bills | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	30	55
U.S. treasury bills | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	30	55
U.S. treasury bills | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	0	0
U.S. treasury bills | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	0	0
Money market funds | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	205	527
Money market funds | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	205	527
Money market funds | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	0	0
Money market funds | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fiduciary Assets:
Fiduciary assets	0	0
Other Assets | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	69	61
Mutual funds	179	192
Other equity investments	8	8
Other Assets | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	69	61
Mutual funds	179	192
Other equity investments	0	0
Other Assets | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Other equity investments	8	8
Other Assets | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Other equity investments	0	0
Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	82	425
Cash and Cash Equivalents | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	82	425
Cash and Cash Equivalents | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	0	0
Cash and Cash Equivalents | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Money market funds	0	0
Other Receivables | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration asset	2	5
Other Receivables | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration asset	0	0
Other Receivables | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration asset	0	0
Other Receivables | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Financial instruments owned:
Contingent purchase consideration asset	2	5
Accounts Payable and Accrued Liabilities and Other Liabilities | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liability	358	352
Accounts Payable and Accrued Liabilities and Other Liabilities | Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liability	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liability	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Liabilities:
Contingent purchase consideration liability	$ 358	$ 352