Risk Rating Under the Risk Rating System (Detail) - Commitments to Extend Credit - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Commitments	$ 66,908	$ 66,884
Pass
Line of Credit Facility [Line Items]
Commitments	65,672	65,645
Risk Level, Special Mention
Line of Credit Facility [Line Items]
Commitments	585	647
Risk Level, Substandard
Line of Credit Facility [Line Items]
Commitments	648	$ 592
Risk Level, Doubtful
Line of Credit Facility [Line Items]
Commitments	$ 3