Investment securities - Changes in Credit Losses Associated with Debt Securities for Which OTTI Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Investments Debt And Equity Securities [Abstract]
Beginning balance	$ 187,114	$ 267,473	$ 197,809	$ 285,399
Additions for credit losses not previously recognized		16,173	9,800	27,659
Reductions for realized losses	(186,320)	(19,449)	(206,815)	(48,861)
Ending balance	$ 794	$ 264,197	$ 794	$ 264,197