Acquisitions (Details) (USD $)
12 Months Ended 0 Months Ended
Dec. 31, 2014
Jan. 08, 2015
Dec. 31, 2013
Business Acquisition      
Consolidated assets $ 3,665,607,000   $ 3,539,575,000
Total gross loans 2,415,690,000   2,295,300,000
Total deposits 2,900,848,000   2,869,138,000
Herget Financial and Herget Bank      
Business Acquisition      
Business acquisition expenses 400,000    
Herget Financial and Herget Bank | Subsequent event      
Business Acquisition      
Percentage of outstanding common stock to be acquired   100.00%  
Aggregate cash consideration   34,100,000  
Per share cash consideration entitled (in dollars per share)   588.00  
Maximum period of measurement adjustments from the closing date of acquisition   1 year  
Herget Bank | Subsequent event      
Business Acquisition      
Number of branches   3  
Herget Financial and Herget Bank      
Business Acquisition      
Consolidated assets 278,800,000    
Total investments 113,000,000    
Total gross loans 112,600,000    
Total deposits $ 246,400,000