Bank Premises and Equipment (Parenthetical) (Detail) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Bank Premises And Equipment [Line Items]
Land and improvements	[1]	$ 682	$ 685
Branches and undeveloped parcels of land
Bank Premises And Equipment [Line Items]
Land and improvements		$ 101	$ 102

[1] At March 31, 2016 and December 31, 2015, land and improvements included $101 and $102, respectively, associated with parcels of undeveloped land intended for future branch expansion.