Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Financial instruments owned:
Total assets measured at fair value	$ 297	$ 330
Fiduciary Assets:
Total fiduciary assets measured at fair value	67	201
Liabilities:
Total liabilities measured at fair value	383	377
Identical Assets (Level 1)
Financial instruments owned:
Total assets measured at fair value	297	314
Fiduciary Assets:
Total fiduciary assets measured at fair value	67	201
Liabilities:
Total liabilities measured at fair value	0	0
Observable Inputs (Level 2)
Financial instruments owned:
Total assets measured at fair value	0	13
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	0	0
Unobservable Inputs (Level 3)
Financial instruments owned:
Total assets measured at fair value	0	3
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	383	377
U.S. treasury bills
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
U.S. treasury bills | Identical Assets (Level 1)
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
U.S. treasury bills | Observable Inputs (Level 2)
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
U.S. treasury bills | Unobservable Inputs (Level 3)
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Money market funds
Fiduciary Assets:
Total fiduciary assets measured at fair value	67	201
Money market funds | Identical Assets (Level 1)
Fiduciary Assets:
Total fiduciary assets measured at fair value	67	201
Money market funds | Observable Inputs (Level 2)
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Money market funds | Unobservable Inputs (Level 3)
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Other Assets
Financial instruments owned:
Exchange traded equity securities	5	6
Mutual funds	162	162
Other equity investment	0	13
Other Assets | Identical Assets (Level 1)
Financial instruments owned:
Exchange traded equity securities	5	6
Mutual funds	162	162
Other equity investment	0	0
Other Assets | Observable Inputs (Level 2)
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Other equity investment	0	13
Other Assets | Unobservable Inputs (Level 3)
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Other equity investment	0	0
Cash and Cash Equivalents
Financial instruments owned:
Money market funds	130	146
Cash and Cash Equivalents | Identical Assets (Level 1)
Financial instruments owned:
Money market funds	130	146
Cash and Cash Equivalents | Observable Inputs (Level 2)
Financial instruments owned:
Money market funds	0	0
Cash and Cash Equivalents | Unobservable Inputs (Level 3)
Financial instruments owned:
Money market funds	0	0
Other Receivables
Financial instruments owned:
Contingent purchase consideration assets	0	3
Other Receivables | Identical Assets (Level 1)
Financial instruments owned:
Contingent purchase consideration assets	0	0
Other Receivables | Observable Inputs (Level 2)
Financial instruments owned:
Contingent purchase consideration assets	0	0
Other Receivables | Unobservable Inputs (Level 3)
Financial instruments owned:
Contingent purchase consideration assets	0	3
Accounts Payable and Accrued Liabilities and Other Liabilities
Liabilities:
Contingent purchase consideration liabilities	383	377
Accounts Payable and Accrued Liabilities and Other Liabilities | Identical Assets (Level 1)
Liabilities:
Contingent purchase consideration liabilities	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Observable Inputs (Level 2)
Liabilities:
Contingent purchase consideration liabilities	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Unobservable Inputs (Level 3)
Liabilities:
Contingent purchase consideration liabilities	$ 383	$ 377