Goodwill and Acquired Intangible Assets (Finite-Lived and Indefinite-Lived Intangible Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 1,542	$ 1,518
Accumulated Amortization	(574)	(501)
Net Amount	968	1,017
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets	1,250	1,232
Total intangible assets	2,792	2,750
Intangible assets, net	2,218	2,249
Exchange and clearing registrations
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets	1,257	1,257
Trade names
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets	121	121
Licenses
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets	52	52
Foreign currency translation adjustment
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets	(180)	(198)
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Amount	76	63
Accumulated Amortization	(21)	(19)
Net Amount	55	44
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Amount	1,599	1,596
Accumulated Amortization	(624)	(532)
Net Amount	975	1,064
Other
Finite-Lived Intangible Assets [Line Items]
Gross Amount	18	18
Accumulated Amortization	(6)	(5)
Net Amount	12	13
Foreign currency translation adjustment
Finite-Lived Intangible Assets [Line Items]
Gross Amount	(151)	(159)
Accumulated Amortization	77	55
Net Amount	$ (74)	$ (104)