Investment securities - Changes in Credit Losses Associated with Debt Securities for Which Other-Than-Temporary Impairment Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	$ 264,197	$ 298,141	$ 285,399	$ 327,912
Additions for credit losses not previously recognized	5,672	9,642	33,331	52,213
Reductions for increases in cash flows		(90)		(5,110)
Reductions for realized losses	(67,926)	(23,127)	(116,787)	(90,449)
Ending balance	$ 201,943	$ 284,566	$ 201,943	$ 284,566