Pledged Assets - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 28,000,000,000		$ 28,000,000,000		$ 27,200,000,000
Fair value of accepted collateral	500,000,000		500,000,000		35,400,000
Amount of securities repledged or sold collateral	0		0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	900,000,000	1,000,000,000	900,000,000	1,100,000,000
US Government Corporations and Agencies Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	19,200,000,000		19,200,000,000		19,000,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	267,700,000		267,700,000		334,000,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	8,500,000,000		8,500,000,000		7,900,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	5,000,000,000		5,000,000,000		3,400,000,000
Available for Sale Investment Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 775,900,000		$ 775,900,000		$ 690,100,000