Intangible Assets (Intangible Assets) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets
Gross Carrying Amount	$ 74	$ 69
Accumulated Amortization	(46)	(42)
Net Carrying Amount	28	27
Core Deposits
Finite-Lived Intangible Assets
Gross Carrying Amount	34	34
Accumulated Amortization	(30)	(29)
Net Carrying Amount	4	5
Other Intangible Assets
Finite-Lived Intangible Assets
Gross Carrying Amount	6	6
Accumulated Amortization	(3)	(3)
Net Carrying Amount	3	3
Noncompete Agreements
Finite-Lived Intangible Assets
Gross Carrying Amount	14	13
Accumulated Amortization	(11)	(10)
Net Carrying Amount	3	3
Customer Relationships
Finite-Lived Intangible Assets
Gross Carrying Amount	20	16
Accumulated Amortization	(2)	0
Net Carrying Amount	$ 18	$ 16