Pledged Assets - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 32,900,000,000		$ 32,900,000,000		$ 32,300,000,000
Fair value of accepted collateral	1,000,000,000		1,000,000,000		1,000,000,000
Amount of securities repledged or sold collateral	0		0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,700,000,000	$ 1,300,000,000	1,600,000,000	$ 1,200,000,000
Us Government Agencies Debt Securities And Other Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	23,700,000,000		23,700,000,000		23,100,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	103,900,000		103,900,000		122,900,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	9,100,000,000		9,100,000,000		9,100,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	5,100,000,000		5,100,000,000		5,900,000,000
Available for Sale Investment Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 17,900,000		$ 17,900,000		$ 884,800,000