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Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value $ 2,415,856us-gaap_AvailableForSaleSecurities $ 2,057,723us-gaap_AvailableForSaleSecurities
Corporate Bonds [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 94,065us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember		  
U.S. Treasury Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 520us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Obligations of U.S. Government Sponsored-Enterprises and Agencies [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 129,755us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 138,088us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Obligations of State and Political Subdivisions [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 1,167,628us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 991,269us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Residential Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 885,211us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 710,620us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
Commercial Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 133,984us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 108,706us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Other Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 4,693us-gaap_AvailableForSaleSecurities
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_CollateralizedSecuritiesOtherMember		  
Level 1 Inputs [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 64,668us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		  
Level 1 Inputs [Member] | U.S. Treasury Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 520us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Level 1 Inputs [Member] | Obligations of State and Political Subdivisions [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 59,455us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Level 1 Inputs [Member] | Other Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 4,693us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_CollateralizedSecuritiesOtherMember		  
Level 2 Inputs [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 2,351,188us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		  
Level 2 Inputs [Member] | Corporate Bonds [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 94,065us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember		  
Level 2 Inputs [Member] | Obligations of U.S. Government Sponsored-Enterprises and Agencies [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 129,755us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Level 2 Inputs [Member] | Obligations of State and Political Subdivisions [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 1,108,173us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Level 2 Inputs [Member] | Residential Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value 885,211us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		  
Level 2 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Available-for-sale investment securities, Total Fair Value $ 133,984us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember