Investment securities - Changes in Credit Losses Associated with Debt Securities for Which OTTI Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Investments Debt And Equity Securities [Abstract]
Beginning balance	$ 197,809	$ 285,399
Additions for credit losses not previously recognized	9,800	11,486
Reductions for realized losses	(20,495)	(29,412)
Ending balance	$ 187,114	$ 267,473