Intangible Assets (Intangible Assets) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets
Gross Carrying Amount	$ 69	$ 49
Accumulated Amortization	(42)	(40)
Net Carrying Amount	27	9
Core Deposits
Finite-Lived Intangible Assets
Gross Carrying Amount	34	34
Accumulated Amortization	(29)	(27)
Net Carrying Amount	5	7
Customer Relationships
Finite-Lived Intangible Assets
Gross Carrying Amount	16
Accumulated Amortization	0
Net Carrying Amount	16
Noncompete Agreements
Finite-Lived Intangible Assets
Gross Carrying Amount	13	10
Accumulated Amortization	(10)	(10)
Net Carrying Amount	3	0
Other Intangible Assets
Finite-Lived Intangible Assets
Gross Carrying Amount	6	5
Accumulated Amortization	(3)	(3)
Net Carrying Amount	$ 3	$ 2