Investment securities - Additional Information (Detail) (USD $)	9 Months Ended		3 Months Ended	9 Months Ended			9 Months Ended
	Sep. 30, 2013 Investment	Dec. 31, 2012	Sep. 30, 2012 Privately Issued Mortgage-Backed Securities [Member]	Sep. 30, 2013 Privately Issued Mortgage-Backed Securities [Member]	Sep. 30, 2012 Privately Issued Mortgage-Backed Securities [Member]	Sep. 30, 2013 Securities Backed by Trust Preferred Securities [Member]	Sep. 30, 2013 Visa Class B [Member]	Sep. 30, 2013 MasterCard Class B [Member]
Investment Holdings [Line Items]
Gross realized gains on investment securities	$ 116,000,000
Gain on sale of holdings							90,000,000	13,000,000
Gross realized losses on investment securities	60,000,000
Privately issued mortgage backed securities sold				1,000,000,000
Net loss on securities sold				46,000,000
Pre-tax other-than-temporary impairment losses			6,000,000	10,000,000	33,000,000
Number of investment securities with aggregate gross unrealized losses	299
Unrealized losses on individual investment securities	109,251,000	252,179,000
Unrealized losses on privately issued residential mortgage-backed securities	57,000,000
Investment securities owned at cost				225,000,000		132,000,000
Unrealized losses on trust preferred securities issued by financial institutions and securities backed by trust preferred securities	21,000,000
Cost Method Investment Securities	$ 298,000,000