Derivative financial instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Increase in net interest income due to interest rate swap agreements	$ 36	$ 38	$ 42
Notional value of interest rate swap agreement impacting net interest income, contract entered into for interest rate risk management purposes	900	900	1,010
Notional value of foreign currency and other option and futures contracts entered into for trading purposes	869	1,400
Net unrealized pre-tax gains related to hedged loans held for sale, commitments to originate loans for sale and commitments to sell loans	89	12
Aggregate fair value of derivative financial instruments in a net liability position	281
Fair value of collateral already posted for derivative financial instruments	266
Fair value of additional collateral to be posted for derivative financial instruments	19
Interest rate swap counterparty collateral postings	69
Credit Risk Derivative [Member]
Derivative [Line Items]
Aggregate fair value of derivative financial instruments in a net liability position	84
Fair value of collateral already posted for derivative financial instruments	65
Interest rate swap maturity 2016 [Member]
Derivative [Line Items]
Interest rate swap agreement maturity date	2016
Interest rate swap maturity 2017[ Member]
Derivative [Line Items]
Interest rate swap agreement maturity date	2017
Interest rate contracts [Member]
Derivative [Line Items]
Notional value of interest rate contracts entered into trading purposes	$ 15,500	$ 13,900