Risk Rating Under the Risk Rating System (Detail) - Commitments to Extend Credit - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Commitments	$ 66,661	$ 63,827
Pass
Line of Credit Facility [Line Items]
Commitments	65,596	62,787
Risk Level, Special Mention
Line of Credit Facility [Line Items]
Commitments	539	660
Risk Level, Substandard
Line of Credit Facility [Line Items]
Commitments	$ 526	$ 380