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Mergers and acquisitions - Additional Information (Details)
$ in Thousands
 3 Months Ended 6 Months Ended
Feb. 08, 2017
USD ($)
shares
Sep. 18, 2015
USD ($)
Jun. 30, 2017
USD ($)
BankingOffice
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
Branch
BankingOffice
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Business Acquisition [Line Items]              
Assets     $ 3,346,570 $ 2,917,958 $ 3,346,570 $ 2,917,958 $ 3,276,881
Deposits     2,727,593   2,727,593   $ 2,671,562
Merger and conversion     767 1,540 $ 1,254 2,146  
Clayton Banks              
Business Acquisition [Line Items]              
Business acquisition, date of acquisition agreement         Feb. 08, 2017    
Acquisition purchase price $ 236,484            
Business acquisition, shares issued | shares 1,521,200            
Cash purchase price $ 184,200            
Assets     1,200,000   $ 1,200,000    
Loans     1,100,000   1,100,000    
Deposits     $ 900,000   $ 900,000    
Number of banking offices | BankingOffice     18   18    
Merger and conversion     $ 767   $ 1,254    
Northwest Georgia Bank              
Business Acquisition [Line Items]              
Business acquisition, date of acquisition agreement         Apr. 27, 2015    
Cash purchase price   $ 1,500          
Merger and conversion       $ 1,540   $ 2,146  
Business acquisition, effective date of acquisition         Sep. 18, 2015    
Number of branches operated prior to acquisition | Branch         6    
Business combination, bargain purchase gain   2,794          
Core deposit intangible asset   $ 4,931          
Estimated useful life of intangible assets   10 years