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Impairment of Investment Securities - Cumulative Roll Forward of Credit Losses Recognized in Earnings for Debt Securities Held and Not Intended to be Sold (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Impairment Of Investment Securities [Line Items]      
Balance, beginning $ 44,736 $ 44,850 $ 36,161
Credit losses on debt securities for which other-than-temporary impairment was not previously recognized 0 0 0
Additional credit losses on debt securities for which other-than- temporary impairment was previously recognized 0 0 8,689
Increases in cash flows expected to be collected, recognized over the remaining life of the security (1,462) (114) 0
Balance, ending $ 43,274 $ 44,736 $ 44,850