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Regulatory & Capital Matters (Details)
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 9.72% 10.08%
Risk-based capital ratio (as a percent) 12.57% 12.93%
Old Second National Bank    
Regulatory & Capital Matters    
Tier 1 capital leverage ratio (as a percent) 11.05% 10.79%
Risk-based capital ratio (as a percent) 14.16% 13.78%
Tier One leverage ratio, basis point increase (as a percent) 0.26%  
Risk-based capital ratio, basis point decrease (as a percent) 0.38%  
Minimum | Old Second National Bank    
Regulatory & Capital Matters    
Tier 1 capital leverage ratio, board-designated threshold 8.00%  
Total Capital Ratio, board-designated threshold (as a percent) 12.00%