Securities - Additional Information (Detail) (USD $)			12 Months Ended				12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012 Collateralized loan obligations (CLOs) [Member]	Dec. 31, 2012 Public and trust department [Member]	Dec. 31, 2011 Public and trust department [Member]	Dec. 31, 2012 Borrowings [Member]	Dec. 31, 2012 Derivative [Member]	Dec. 31, 2012 Other debt securities [Member]	Dec. 31, 2011 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
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
Securities available for sale pledged as collateral	8,100,000,000		6,600,000,000		900,000,000	600,000,000
Permitted by contract to sell or repledge			19,000,000	12,000,000
Repledged to secure public agency or bankruptcy deposits and to cover short sales			$ 0	$ 12,000,000