Risk Rating Under the Risk Rating System (Detail) (Commitments to Extend Credit, USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Commitments	$ 64,425	$ 63,827
Pass
Line of Credit Facility [Line Items]
Commitments	63,538	62,787
Risk Level, Special Mention
Line of Credit Facility [Line Items]
Commitments	447	660
Risk Level, Substandard
Line of Credit Facility [Line Items]
Commitments	$ 440	$ 380