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Regulatory & Capital Matters (Details)
 9 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 8.46% 9.93%
Total capital ratio (as a percent) 15.36% 17.68%
Old Second National Bank    
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 11.26% 12.02%
Tier One leverage ratio, basis point increase (as a percent) 0.76%  
Total capital ratio (as a percent) 17.10% 18.73%
Risk-based capital ratio, basis point decrease (as a percent) 1.63%  
Old Second National Bank | Minimum    
Regulatory & Capital Matters    
Tier 1 capital leverage ratio, board-designated threshold 8.00%  
Total Capital Ratio, board-designated threshold (as a percent) 12.00%