Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial instruments owned:
Total assets measured at fair value	$ 908	$ 317
Fiduciary Assets:
Total fiduciary assets measured at fair value	323	400
Liabilities:
Total liabilities measured at fair value	243	225
Identical Assets (Level 1)
Financial instruments owned:
Total assets measured at fair value	832	225
Fiduciary Assets:
Total fiduciary assets measured at fair value	323	400
Liabilities:
Total liabilities measured at fair value	0	0
Observable Inputs (Level 2)
Financial instruments owned:
Total assets measured at fair value	8	8
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	68	84
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	243	225
Senior Debt Obligations Due 2014
Liabilities:
Acquisition related derivative contracts	0	0
Senior Debt Obligations Due 2014 | Identical Assets (Level 1)
Liabilities:
Acquisition related derivative contracts	0	0
Senior Debt Obligations Due 2014 | Observable Inputs (Level 2)
Liabilities:
Acquisition related derivative contracts	0	0
Senior Debt Obligations Due 2014 | Unobservable Inputs (Level 3)
Liabilities:
Acquisition related derivative contracts	0	0
Money market funds
Fiduciary Assets:
Total fiduciary assets measured at fair value	173	360
Money market funds | Identical Assets (Level 1)
Fiduciary Assets:
Total fiduciary assets measured at fair value	173	360
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
U.S. Treasury Bills
Fiduciary Assets:
Total fiduciary assets measured at fair value	150	40
U.S. Treasury Bills | Identical Assets (Level 1)
Fiduciary Assets:
Total fiduciary assets measured at fair value	150	40
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
Other Assets
Financial instruments owned:
Exchange traded equity securities	59	4
Mutual funds	186	166
Other equity investment	8	8
Contingent purchase consideration asset	68	84
Other Assets | Identical Assets (Level 1)
Financial instruments owned:
Exchange traded equity securities	59	4
Mutual funds	186	166
Other equity investment	0	0
Contingent purchase consideration asset	0	0
Other Assets | Observable Inputs (Level 2)
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Other equity investment	8	8
Contingent purchase consideration asset	0	0
Other Assets | Unobservable Inputs (Level 3)
Financial instruments owned:
Exchange traded equity securities	0	0
Mutual funds	0	0
Other equity investment	0	0
Contingent purchase consideration asset	68	84
Cash and Cash Equivalents
Financial instruments owned:
Money market funds	587	55
Cash and Cash Equivalents | Identical Assets (Level 1)
Financial instruments owned:
Money market funds	587	55
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
Accounts Payable and Accrued Liabilities and Other Liabilities
Liabilities:
Contingent purchase consideration liability	243	225
Accounts Payable and Accrued Liabilities and Other Liabilities | Identical Assets (Level 1)
Liabilities:
Contingent purchase consideration liability	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Observable Inputs (Level 2)
Liabilities:
Contingent purchase consideration liability	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Unobservable Inputs (Level 3)
Liabilities:
Contingent purchase consideration liability	$ 243	$ 225