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Securities - Roll-Forward of Other-Than-Temporary Impairment Amount Related to Credit Losses (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Marketable Securities [Abstract]      
Balance of credit losses on debt securities for which a portion of other-than-temporary impairment was recognized in earnings, beginning of period $ 4,054 $ 4,054 $ 4,054
Credit losses previously recognized on securities sold during the period (1,983) 0 0
Additional credit loss for which other-than-temporary impairment was not previously recognized 0 0 0
Additional credit loss for which other-than-temporary impairment was previously recognized 0 0 0
Balance of credit losses on debt securities for which a portion of other-than-temporary impairment was recognized in earnings, end of period $ 2,071 $ 4,054 $ 4,054