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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Investment Securities [Line Items]    
Amortized Cost $ 1,317,318us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 1,350,466us-gaap_AvailableForSaleSecuritiesAmortizedCost
Estimated Fair Value of Available for Sale 1,309,819us-gaap_AvailableForSaleSecurities 1,318,365us-gaap_AvailableForSaleSecurities
US Government Agencies Debt Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 23,300us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Estimated Fair Value of Available for Sale 25,900us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Obligations of U.S. Government-Sponsored Enterprises [Member]    
Investment Securities [Line Items]    
Amortized Cost 947,700us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		  
Estimated Fair Value of Available for Sale $ 951,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember