Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Financial instruments owned:
Total assets measured at fair value	$ 317,000,000	$ 410,000,000
Fiduciary Assets:
Total fiduciary assets measured at fair value	400,000,000	100,000,000
Liabilities:
Total liabilities measured at fair value	225,000,000	624,000,000
Identical Assets (Level 1)
Financial instruments owned:
Total assets measured at fair value	225,000,000	402,000,000
Fiduciary Assets:
Total fiduciary assets measured at fair value	400,000,000	100,000,000
Liabilities:
Total liabilities measured at fair value	0	0
Observable Inputs (Level 2)
Financial instruments owned:
Total assets measured at fair value	8,000,000	8,000,000
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	0	116,000,000
Unobservable Inputs (Level 3)
Financial instruments owned:
Total assets measured at fair value	84,000,000	0
Fiduciary Assets:
Total fiduciary assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	225,000,000	508,000,000
Other Assets
Financial instruments owned:
Exchange traded equity securities	4,000,000	133,000,000
Mutual funds	166,000,000	151,000,000
Contingent purchase consideration asset	84,000,000	0
Other Assets | Identical Assets (Level 1)
Financial instruments owned:
Exchange traded equity securities	4,000,000	133,000,000
Mutual funds	166,000,000	151,000,000
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
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
Contingent purchase consideration asset	84,000,000	0
Cash and Cash Equivalents
Financial instruments owned:
Money market funds	55,000,000	118,000,000
Cash and Cash Equivalents | Identical Assets (Level 1)
Financial instruments owned:
Money market funds	55,000,000	118,000,000
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:
Other equity investment	8,000,000	8,000,000
Other Receivables | Identical Assets (Level 1)
Financial instruments owned:
Other equity investment	0	0
Other Receivables | Observable Inputs (Level 2)
Financial instruments owned:
Other equity investment	8,000,000	8,000,000
Other Receivables | Unobservable Inputs (Level 3)
Financial instruments owned:
Other equity investment	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities
Liabilities:
Contingent purchase consideration liability	225,000,000	183,000,000
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	225,000,000	183,000,000
Money market funds
Fiduciary Assets:
Total fiduciary assets measured at fair value	360,000,000	80,000,000
Money market funds | Identical Assets (Level 1)
Fiduciary Assets:
Total fiduciary assets measured at fair value	360,000,000	80,000,000
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	40,000,000	20,000,000
U.S. Treasury Bills | Identical Assets (Level 1)
Fiduciary Assets:
Total fiduciary assets measured at fair value	40,000,000	20,000,000
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
Senior Debt Obligations Due 2014 | Long-term Debt
Liabilities:
Acquisition related derivative contracts	0	441,000,000
Senior Debt Obligations Due 2014 | Long-term Debt | Identical Assets (Level 1)
Liabilities:
Acquisition related derivative contracts	0	0
Senior Debt Obligations Due 2014 | Long-term Debt | Observable Inputs (Level 2)
Liabilities:
Acquisition related derivative contracts	0	116,000,000
Senior Debt Obligations Due 2014 | Long-term Debt | Unobservable Inputs (Level 3)
Liabilities:
Acquisition related derivative contracts	$ 0	$ 325,000,000