Intangible Assets - Schedule of Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets
Gross Carrying Amount	$ 272	$ 273
Accumulated Amortization	(145)	(134)
Net Carrying Amount	127	139
Core deposit intangibles
Finite-Lived Intangible Assets
Gross Carrying Amount	229	229
Accumulated Amortization	(126)	(116)
Net Carrying Amount	103	113
Customer relationships
Finite-Lived Intangible Assets
Gross Carrying Amount	24	24
Accumulated Amortization	(6)	(5)
Net Carrying Amount	18	19
Operating leases
Finite-Lived Intangible Assets
Gross Carrying Amount	16	17
Accumulated Amortization	(12)	(12)
Net Carrying Amount	4	5
Other
Finite-Lived Intangible Assets
Gross Carrying Amount	3	3
Accumulated Amortization	(1)	(1)
Net Carrying Amount	$ 2	$ 2