Investment securities - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended		3 Months Ended			6 Months Ended
	Jun. 30, 2013 Investment	Jun. 30, 2013 Investment	Dec. 31, 2012	Jun. 30, 2013 Visa Class B [Member]	Jun. 30, 2013 MasterCard Class B [Member]	Jun. 30, 2012 Privately Issued Mortgage-Backed Securities [Member]	Jun. 30, 2013 Privately Issued Mortgage-Backed Securities [Member]	Jun. 30, 2012 Privately Issued Mortgage-Backed Securities [Member]
Investment Holdings [Line Items]
Gross realized gains on investment securities	$ 116,000,000	$ 116,000,000
Gain on sale of holdings				90,000,000	13,000,000
Gross realized losses on investment securities	60,000,000	60,000,000
Privately issued mortgage backed securities sold	1,000,000,000	1,000,000,000
Net loss on secuirities sold	46,000,000
Pre-tax other-than-temporary impairment losses						16,000,000	10,000,000	28,000,000
Number of investment securities with aggregate gross unrealized losses	278	278
Unrealized losses on individual investment securities	122,540,000	122,540,000	252,179,000
Unrealized losses on privately issued residential mortgage-backed securities		68,000,000
Residential mortgage-backed securities at cost	231,000,000	231,000,000
Unrealized losses on trust preferred securities issued by financial institutions and securities backed by trust preferred securities		22,000,000
Trust preferred securities issued by financial institutions and securities backed by trust preferred securities cost	135,000,000	135,000,000
Cost Method Investment Securities	$ 306,000,000	$ 306,000,000