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Regulatory & Capital Matters (Details)
 3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 8.84% 8.90%
Risk-based capital ratio (as a percent) 12.11% 12.29%
Old Second National Bank    
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 10.14% 10.24%
Risk-based capital ratio (as a percent) 13.33% 13.45%
Tier One leverage ratio, basis point increase (as a percent) 0.10%  
Risk-based capital ratio, basis point decrease (as a percent) 0.12%  
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%