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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
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Impairment of Investment Securities Disclosure [Abstract]    
Balance, beginning (a) [1] $ 17,056 $ 24,851
Credit losses on debt securities for which other-than-temporary impairment was not previously recognized 0 0
Additional credit losses on debt securities for which other-than-temporary impairment was previously recognized 0 0
Balance, ending 16,828 24,590
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows 0 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease) $ 228 $ 261
[1] The beginning balance represents credit related losses included in other-than-temporary impairment charges recognized on debt securities in prior periods.