Securities - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	$ 4,400,000,000
Collateralized loan obligations (CLOs) [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Carrying amount of the CLOs	1,100,000,000
Unrealized losses remaining in OCI, net of amortization	301,000,000
Recognized losses in OCI	62,000,000
Borrowings [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	1,000,000,000
Derivative [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	600,000,000
Public And Trust Department [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale pledged as collateral	2,800,000,000
Permitted by contract to sell or repledge	7,000,000	12,000,000
Repledged to secure public agency or bankruptcy deposits and to cover short sales	0	12,000,000
Other debt securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Tax exempt conduit debt bonds	$ 900,000,000	$ 900,000,000