Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 275	$ 87
Accumulated Amortization	(74)	(47)
Net Carrying Amount	201	40
Core Deposits
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	229	34
Accumulated Amortization	(57)	(30)
Net Carrying Amount	172	4
Other Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	4	7
Accumulated Amortization	(1)	(3)
Net Carrying Amount	3	4
Noncompete Agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	13	14
Accumulated Amortization	(11)	(11)
Net Carrying Amount	2	3
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	29	32
Accumulated Amortization	(5)	(3)
Net Carrying Amount	$ 24	$ 29