Intangible Assets (Intangible Assets) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets
Gross Carrying Amount	$ 73	$ 69
Accumulated Amortization	(43)	(42)
Net Carrying Amount	30	27
Core Deposits
Finite-Lived Intangible Assets
Gross Carrying Amount	34	34
Accumulated Amortization	(29)	(29)
Net Carrying Amount	5	5
Other Intangible Assets
Finite-Lived Intangible Assets
Gross Carrying Amount	6	6
Accumulated Amortization	(2)	(3)
Net Carrying Amount	4	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	19	16
Accumulated Amortization	(1)	0
Net Carrying Amount	$ 18	$ 16