Summary of Significant Accounting Policies (Premises and Equipment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Accumulated Depreciation and Amortization	$ 36	$ 27
Depreciation and Amortization Expense	14	9	$ 6
Property, Plant and Equipment, Net	$ 66	$ 76
Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	40 years