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Investment securities - Changes in Credit Losses Associated with Debt Securities for Which OTTI Losses (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Investments Debt And Equity Securities [Abstract]        
Beginning balance $ 794 $ 264,197 $ 197,809 $ 285,399
Additions for credit losses not previously recognized    5,672 9,800 33,331
Reductions for realized losses (626) (67,926) (207,441) (116,787)
Ending balance $ 168 $ 201,943 $ 168 $ 201,943