Acquisition of MainStreet Bankshares, Inc. - (Details)
9 Months Ended
Jan. 01, 2015
USD ($)
bank
$ / shares
shares
Sep. 30, 2016
USD ($)
shares
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
[1]
Business Acquisition [Line Items]        
Number of shares granted in conjunction with the acquisition (in shares) | shares   0    
Net loans   $ 1,070,444,000   $ 992,924,000
Assets   1,615,534,000 $ 1,512,437,000 1,547,599,000
Deposits   1,306,008,000   $ 1,262,660,000
MainStreet BankShares, Inc. [Member]        
Business Acquisition [Line Items]        
Net loans $ 122,000,000      
Assets 164,000,000      
Deposits $ 137,000,000      
Franklin Community Bank, N.A. [Member] | Virginia [Member]        
Business Acquisition [Line Items]        
Number of banking offices served (banks) | bank 3      
MainStreet BankShares, Inc. [Member]        
Business Acquisition [Line Items]        
Shares received by acquiree entity in exchange of acquirer entity's common stock (in dollars per share) | $ / shares $ 3.46      
Shares received by acquiree entity in exchange of acquirer entity's common stock (in shares) | shares 0.482      
Share exchange ratio on acquisitions 0.643      
Cash dividends received for purchase of acquisition $ 6,000,000      
Amount of loan taken to fund merger $ 0      
Number of shares granted in conjunction with the acquisition (in shares) | shares 43,086      
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]        
Net impact to income before taxes   463,000 1,142,000  
MainStreet BankShares, Inc. [Member] | Amortization of intangible asset [Member]        
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]        
Intangibles   (185,000) (220,000)  
MainStreet BankShares, Inc. [Member] | CD [Member] | Interest on deposits [Member]        
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]        
Financial assets   0 216,000  
MainStreet BankShares, Inc. [Member] | Brokered CD [Member] | Interest on deposits [Member]        
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]        
Financial assets   0 2,000  
MainStreet BankShares, Inc. [Member] | Acquired Impaired Loans [Member] | Interest and fees on loans [Member]        
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]        
Financial assets   445,000 827,000  
MainStreet BankShares, Inc. [Member] | Performing [Member] | Interest and fees on loans [Member]        
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]        
Financial assets   $ 203,000 $ 317,000  
[1] Derived from audited consolidated financial statements.