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Securities - Roll-Forward of Other-Than-Temporary Impairment Amount Related to Credit Losses (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
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 $ 4,054us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Additional credit loss for which other-than-temporary impairment was not previously recognized 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment
Additional credit loss for which other-than-temporary impairment was previously recognized 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses
Balance of credit losses on debt securities for which a portion of other-than-temporary impairment was recognized in earnings, end of period $ 4,054us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld