Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial instruments owned:
Total assets measured at fair value	$ 301	$ 410
Fiduciary Assets:
Fiduciary assets	347	100
Liabilities:
Total liabilities measured at fair value	196	624
Identical Assets (Level 1)
Financial instruments owned:
Total assets measured at fair value	228	402
Fiduciary Assets:
Fiduciary assets	347	100
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:
Fiduciary assets	0	0
Liabilities:
Total liabilities measured at fair value	0	116
Unobservable Inputs (Level 3)
Financial instruments owned:
Total assets measured at fair value	65	0
Fiduciary Assets:
Fiduciary assets	0	0
Liabilities:
Total liabilities measured at fair value	196	508
US Treasury Bills
Fiduciary Assets:
Fiduciary assets	0	20
US Treasury Bills | Identical Assets (Level 1)
Fiduciary Assets:
Fiduciary assets	0	20
US Treasury Bills | Observable Inputs (Level 2)
Fiduciary Assets:
Fiduciary assets	0	0
US Treasury Bills | Unobservable Inputs (Level 3)
Fiduciary Assets:
Fiduciary assets	0	0
Money Market Funds
Fiduciary Assets:
Fiduciary assets	347	80
Money Market Funds | Identical Assets (Level 1)
Fiduciary Assets:
Fiduciary assets	347	80
Money Market Funds | Observable Inputs (Level 2)
Fiduciary Assets:
Fiduciary assets	0	0
Money Market Funds | Unobservable Inputs (Level 3)
Fiduciary Assets:
Fiduciary assets	0	0
Forward Contracts
Liabilities:
Acquisition related derivative contract	0	441
Forward Contracts | Identical Assets (Level 1)
Liabilities:
Acquisition related derivative contract	0	0
Forward Contracts | Observable Inputs (Level 2)
Liabilities:
Acquisition related derivative contract	0	116
Forward Contracts | Unobservable Inputs (Level 3)
Liabilities:
Acquisition related derivative contract	0	325
Other Assets
Financial instruments owned:
Exchange traded equity securities	4	133
Mutual funds	153	151
Other equity investments	8	8
Contingent purchase consideration asset	65	0
Other Assets | Identical Assets (Level 1)
Financial instruments owned:
Exchange traded equity securities	4	133
Mutual funds	153	151
Other equity investments	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 investments	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 investments	0	0
Contingent purchase consideration asset	65	0
Cash and Cash Equivalents
Financial instruments owned:
Money market funds	71	118
Cash and Cash Equivalents | Identical Assets (Level 1)
Financial instruments owned:
Money market funds	71	118
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	196	183
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	$ 196	$ 183