Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Financial instruments owned:
Total assets measured at fair value		$ 241	$ 410
Fiduciary Assets:
Fiduciary assets		60	100
Liabilities:
Total liabilities measured at fair value		454	624
Identical Assets (Level 1)
Financial instruments owned:
Total assets measured at fair value		233	402
Fiduciary Assets:
Fiduciary assets		60	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		0	0
Fiduciary Assets:
Fiduciary assets		0	0
Liabilities:
Total liabilities measured at fair value		454	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		60	80
Money Market Funds | Identical Assets (Level 1)
Fiduciary Assets:
Fiduciary assets		60	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
Other Assets
Financial instruments owned:
Exchange traded equity securities	[1]	21	133
Mutual funds	[1]	145	151
Other equity investments	[1]	8	8
Other Assets | Identical Assets (Level 1)
Financial instruments owned:
Exchange traded equity securities	[1]	21	133
Mutual funds	[1]	145	151
Other equity investments	[1]	0	0
Other Assets | Observable Inputs (Level 2)
Financial instruments owned:
Exchange traded equity securities	[1]	0	0
Mutual funds	[1]	0	0
Other equity investments	[1]	8	8
Other Assets | Unobservable Inputs (Level 3)
Financial instruments owned:
Exchange traded equity securities	[1]	0	0
Mutual funds	[1]	0	0
Other equity investments	[1]	0	0
Cash and Cash Equivalents
Financial instruments owned:
Money market funds	[2]	67	118
Cash and Cash Equivalents | Identical Assets (Level 1)
Financial instruments owned:
Money market funds	[2]	67	118
Cash and Cash Equivalents | Observable Inputs (Level 2)
Financial instruments owned:
Money market funds	[2]	0	0
Cash and Cash Equivalents | Unobservable Inputs (Level 3)
Financial instruments owned:
Money market funds	[2]	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities
Liabilities:
Contingent purchase consideration liability	[3]	171	183
Accounts Payable and Accrued Liabilities and Other Liabilities | Identical Assets (Level 1)
Liabilities:
Contingent purchase consideration liability	[3]	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Observable Inputs (Level 2)
Liabilities:
Contingent purchase consideration liability	[3]	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Unobservable Inputs (Level 3)
Liabilities:
Contingent purchase consideration liability	[3]	171	183
Accounts Payable and Accrued Liabilities | Forward Contracts
Liabilities:
Acquisition related derivative contract		283	441
Accounts Payable and Accrued Liabilities | Forward Contracts | Identical Assets (Level 1)
Liabilities:
Acquisition related derivative contract		0	0
Accounts Payable and Accrued Liabilities | Forward Contracts | Observable Inputs (Level 2)
Liabilities:
Acquisition related derivative contract		0	116
Accounts Payable and Accrued Liabilities | Forward Contracts | Unobservable Inputs (Level 3)
Liabilities:
Acquisition related derivative contract		$ 283	$ 325

[1] Included in other assets in the consolidated balance sheets.

[2] Included in cash and cash equivalents in the consolidated balance sheets.

[3] Included in accounts payable and accrued liabilities and other liabilities in the consolidated balance sheets.