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Goodwill and Other Amortizing Intangible Assets - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Goodwill And Other Intangibles [Line Items]      
Goodwill $ 161,429,000us-gaap_Goodwill $ 159,956,000us-gaap_Goodwill  
Impairment charges on goodwill 0us-gaap_GoodwillImpairmentLoss    
Amortization of other intangible assets $ 631,000us-gaap_AmortizationOfIntangibleAssets $ 1,064,000us-gaap_AmortizationOfIntangibleAssets $ 1,467,000us-gaap_AmortizationOfIntangibleAssets
Core Deposits Intangibles [Member]      
Goodwill And Other Intangibles [Line Items]      
Core deposits remaining amortization period 4 years 10 months 24 days    
Core deposits weighted average amortization period 3 years    
Customer Lists [Member]      
Goodwill And Other Intangibles [Line Items]      
Core deposits weighted average amortization period 14 years 9 months