Securities - Additional Information (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	$ 7,000,000,000
Borrowings [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	700,000,000
Derivative [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	300,000,000
Public and trust deposits [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	6,000,000,000
Permitted by contract to sell or repledge	15,000,000	19,000,000
Repledged to secure public agency or bankruptcy deposits and to cover short sales	$ 0	$ 0