Derivative financial instruments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivative [Line Items]
Increase in net interest income due to interest rate swap agreements	$ 9,000,000	$ 9,000,000	$ 27,000,000	$ 28,000,000
Notional value of foreign currency and other option and future contracts entered into trading purposes	1,200,000,000		1,200,000,000		1,400,000,000
Net unrealized pre-tax gains related to hedged loans held for sale, commitments to originate loans for sale and commitments to sell loans			81,000,000		12,000,000
Aggregate fair value of derivative financial instruments in a net liability position	330,000,000		330,000,000
Collateral already posted aggregate fair value derivative instruments in net liability position	306,000,000		306,000,000
Fair value of collateral already posted for derivative financial instruments	71,000,000		71,000,000
Maximum amount of additional collateral required to post to counterparties	24,000,000		24,000,000
Interest rate swap counterparty collateral postings	77,000,000		77,000,000
Interest rate contracts [Member]
Derivative [Line Items]
Notional value of interest rate contracts entered into trading purposes	15,100,000,000		15,100,000,000		13,900,000,000
Credit Risk Derivative [Member]
Derivative [Line Items]
Aggregate fair value of derivative financial instruments in a net liability position	$ 95,000,000		$ 95,000,000