Investment securities - Changes in Credit Losses Associated with Debt Securities for which Other-Than-Temporary Impairment Losses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Estimated credit losses - beginning balance	$ 285,399	$ 327,912	$ 284,513
Additions for credit losses not previously recognized	47,822	77,035	74,288
Reductions for increases in cash flows		(5,111)	(754)
Reductions for realized losses	(135,412)	(114,437)	(30,135)
Estimated credit losses - ending balance	$ 197,809	$ 285,399	$ 327,912