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Goodwill and Other Amortizing Intangible Assets - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Oct. 01, 2015
Goodwill And Other Intangibles [Line Items]        
Goodwill $ 164,500,000 $ 161,429,000    
Increase in goodwill 3,100,000      
Impairment charges on goodwill 0 0 $ 0  
Total amortization 18,895,000 21,789,000    
Amortization of other intangible assets $ 605,000 631,000 $ 1,064,000  
First Community Bank        
Goodwill And Other Intangibles [Line Items]        
Goodwill       $ 3,070,000
Customer deposit intangibles        
Goodwill And Other Intangibles [Line Items]        
Core deposits remaining amortization period 9 years 9 months 18 days      
Core deposits weighted average amortization period 5 years 1 month 18 days      
Customer deposit intangibles | Previous acquisitions        
Goodwill And Other Intangibles [Line Items]        
Total amortization $ 3,500,000      
Customer deposit intangibles | First Community Bank        
Goodwill And Other Intangibles [Line Items]        
Total amortization $ 200,000      
Customer list intangible        
Goodwill And Other Intangibles [Line Items]        
Core deposits weighted average amortization period 13 years 9 months 18 days      
Total amortization $ 276,000 $ 16,000