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Goodwill and Other Amortizing Intangible Assets - Additional Information (Detail) - USD ($)
 12 Months Ended
Apr. 03, 2017
Dec. 02, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill And Other Intangibles [Line Items]          
Goodwill     $ 255,353,000 $ 186,500,000  
Goodwill, Acquired During Period     68,900,000    
Impairment charges on goodwill     0 0 $ 0
Servicing Asset     75,000    
Total amortization     6,822,000 19,442,000  
Amortization of other intangible assets     $ 3,081,000 547,000 $ 605,000
FirstMerit Bank, NA, Branch Acquisition [Member]          
Goodwill And Other Intangibles [Line Items]          
Goodwill   $ 20,478,000      
Goodwill, Acquired During Period   1,600,000      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles   11,330,000      
DCB Financial Corp [Member]          
Goodwill And Other Intangibles [Line Items]          
Goodwill, Acquired During Period $ 70,600,000        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles 5,998,000        
Customer deposit intangibles          
Goodwill And Other Intangibles [Line Items]          
Core deposits remaining amortization period     9 years 4 months    
Core deposits weighted average amortization period     8 years 11 months    
Customer deposit intangibles | FirstMerit Bank, NA, Branch Acquisition [Member]          
Goodwill And Other Intangibles [Line Items]          
Finite-lived Intangible Assets Acquired   $ 15,700,000      
Customer deposit intangibles | DCB Financial Corp [Member]          
Goodwill And Other Intangibles [Line Items]          
Finite-lived Intangible Assets Acquired 4,700,000        
Customer deposit intangibles | Customer list intangible          
Goodwill And Other Intangibles [Line Items]          
Finite-lived Intangible Assets Acquired $ 1,300,000        
Customer list intangible          
Goodwill And Other Intangibles [Line Items]          
Core deposits remaining amortization period     11 years 8 months    
Core deposits weighted average amortization period     9 years 11 months    
Total amortization     $ 751,000 $ 444,000