Goodwill and Other Intangible Assets - Summary Of Intangible Assets and Accumulated Amortization Included In The Consolidated Statements of Condition (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Intangible Assets [Line Items]
Gross Carrying Amount	$ 449	$ 449
Accumulated Amortization	(138)	(95)
Net Carrying Value	311	354
Core deposit intangibles
Intangible Assets [Line Items]
Gross Carrying Amount	371	371
Accumulated Amortization	(117)	(81)
Net Carrying Value	254	290
Customer relationships
Intangible Assets [Line Items]
Gross Carrying Amount	37	37
Accumulated Amortization	(10)	(8)
Net Carrying Value	27	29
Other
Intangible Assets [Line Items]
Gross Carrying Amount	41	41
Accumulated Amortization	(11)	(6)
Net Carrying Value	$ 30	$ 35