Goodwill and Acquired Intangible Assets (Reclassification to Held-for-Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 2,303	$ 1,589
Accumulated Amortization	(597)	(620)
Net Amount	1,706	969
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Amount	2,049	1,599
Accumulated Amortization	(626)	(648)
Net Amount	1,423	$ 951
Customer Relationships | U.S. Fixed Income business | Disposal group, held-for-sale
Finite-Lived Intangible Assets [Line Items]
Gross Amount	83
Accumulated Amortization	(50)
Net Amount	$ 33