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Regulatory & Capital Matters (Details)
 9 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 9.32% 8.69%
Total capital ratio (as a percent) 15.42% 15.56%
Old Second National Bank    
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 10.65% 9.94%
Tier One leverage ratio, basis point increase (as a percent) 0.71%  
Total capital ratio (as a percent) 16.24% 15.23%
Risk-based capital ratio, basis point decrease (as a percent) 1.01%  
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%