Goodwill and Acquired Intangible Assets (Finite-Lived and Indefinite-Lived Intangible Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 1,493	$ 1,518
Accumulated Amortization	(503)	(501)
Net Amount	990	1,017
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets	1,210	1,232
Total intangible assets	2,703	2,750
Intangible assets, net	2,200	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	(220)	(198)
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Amount	77	63
Accumulated Amortization	(16)	(19)
Net Amount	61	44
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Amount	1,599	1,596
Accumulated Amortization	(576)	(532)
Net Amount	1,023	1,064
Other
Finite-Lived Intangible Assets [Line Items]
Gross Amount	19	18
Accumulated Amortization	(5)	(5)
Net Amount	14	13
Foreign currency translation adjustment
Finite-Lived Intangible Assets [Line Items]
Gross Amount	(202)	(159)
Accumulated Amortization	94	55
Net Amount	$ (108)	$ (104)