Goodwill and Other Intangible Assets - Gross Carrying Amount, Accumulated Amortization And Net Carrying Amount Of Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 4,010	$ 3,894
Accumulated Amortization	(2,123)	(2,067)
Net Carrying Amount	1,887	1,827	$ 2,030
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2,803	2,704
Accumulated Amortization	(1,464)	(1,450)
Net Carrying Amount	1,339	1,254
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	404	393
Accumulated Amortization	(133)	(113)
Net Carrying Amount	271	280
Core deposits
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	700	690
Accumulated Amortization	(444)	(425)
Net Carrying Amount	256	265
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	103	107
Accumulated Amortization	(82)	(79)
Net Carrying Amount	$ 21	$ 28