Derivative financial instruments - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivative [Line Items]
Increase in net interest income due to interest rate swap agreements	$ 10,000,000	$ 9,000,000
Notional value of foreign currency and other option and futures contracts entered into for trading purposes	921,000,000		869,000,000
Net unrealized pre-tax gains related to hedged loans held for sale, commitments to originate loans for sale and commitments to sell loans	62,000,000		89,000,000
Aggregate fair value of derivative financial instruments in a liability position	330,000,000		374,000,000
Net liability positions with counterparties	236,000,000		281,000,000
Post collateral requirements relating to positions	218,000,000		266,000,000
Fair value of additional collateral to be posted for derivative financial instruments	16,000,000
Aggregate fair value of derivative financial instruments in asset position	168,000,000		164,000,000
Net asset positions with counterparties	75,000,000		71,000,000
Counterparties posted collateral relating to positions	75,000,000		69,000,000
Credit Risk Derivative [Member]
Derivative [Line Items]
Aggregate fair value of derivative financial instruments in a net liability position	74,000,000
Fair value of collateral already posted for derivative financial instruments	58,000,000
Interest rate contracts [Member]
Derivative [Line Items]
Notional value of interest rate contracts entered into trading purposes	$ 15,200,000,000		$ 15,500,000,000