Risk Rating Under the Risk Rating System (Detail) - Commitments to Extend Credit - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Commitments	$ 66,884	$ 63,827
Pass
Line of Credit Facility [Line Items]
Commitments	65,645	62,787
Special Mention
Line of Credit Facility [Line Items]
Commitments	647	660
Substandard
Line of Credit Facility [Line Items]
Commitments	$ 592	$ 380