Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Financial instruments owned:
Total assets measured at fair value		$ 311	$ 382
Fiduciary Assets:
Fiduciary assets		69	111
Liabilities:
Total liabilities measured at fair value		268	189
Identical Assets (Level 1)
Financial instruments owned:
Total assets measured at fair value		303	382
Fiduciary Assets:
Fiduciary assets		69	111
Liabilities:
Total liabilities measured at fair value		0	0
Observable Inputs (Level 2)
Financial instruments owned:
Total assets measured at fair value		8	0
Fiduciary Assets:
Fiduciary assets		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	0
Fiduciary Assets:
Fiduciary assets		0	0
Liabilities:
Total liabilities measured at fair value		268	189
Money Market Funds
Fiduciary Assets:
Fiduciary assets		69	111
Money Market Funds | Identical Assets (Level 1)
Fiduciary Assets:
Fiduciary assets		69	111
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:
Foreign exchange forward contracts	[1]	100	0
Forward Contracts | Identical Assets (Level 1)
Liabilities:
Foreign exchange forward contracts	[1]	0	0
Forward Contracts | Observable Inputs (Level 2)
Liabilities:
Foreign exchange forward contracts	[1]	0	0
Forward Contracts | Unobservable Inputs (Level 3)
Liabilities:
Foreign exchange forward contracts	[1]	100	0
Other Assets
Financial instruments owned:
Exchange traded equity securities	[2]	119	81
Mutual funds	[2]	153	158
Other Assets | Identical Assets (Level 1)
Financial instruments owned:
Exchange traded equity securities	[2]	119	81
Mutual funds	[2]	153	158
Other Assets | Observable Inputs (Level 2)
Financial instruments owned:
Exchange traded equity securities	[2]	0	0
Mutual funds	[2]	0	0
Other Assets | Unobservable Inputs (Level 3)
Financial instruments owned:
Exchange traded equity securities	[2]	0	0
Mutual funds	[2]	0	0
Cash and Cash Equivalents
Financial instruments owned:
Money market funds	[3]	31	143
Cash and Cash Equivalents | Identical Assets (Level 1)
Financial instruments owned:
Money market funds	[3]	31	143
Cash and Cash Equivalents | Observable Inputs (Level 2)
Financial instruments owned:
Money market funds	[3]	0	0
Cash and Cash Equivalents | Unobservable Inputs (Level 3)
Financial instruments owned:
Money market funds	[3]	0	0
Other Receivables
Financial instruments owned:
Other equity investments	[2]	8	0
Other Receivables | Identical Assets (Level 1)
Financial instruments owned:
Other equity investments	[2]	0	0
Other Receivables | Observable Inputs (Level 2)
Financial instruments owned:
Other equity investments	[2]	8	0
Other Receivables | Unobservable Inputs (Level 3)
Financial instruments owned:
Other equity investments	[2]	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities
Liabilities:
Contingent purchase consideration liability	[4]	168	189
Accounts Payable and Accrued Liabilities and Other Liabilities | Identical Assets (Level 1)
Liabilities:
Contingent purchase consideration liability	[4]	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Observable Inputs (Level 2)
Liabilities:
Contingent purchase consideration liability	[4]	0	0
Accounts Payable and Accrued Liabilities and Other Liabilities | Unobservable Inputs (Level 3)
Liabilities:
Contingent purchase consideration liability	[4]	$ 168	$ 189

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

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

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

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