Regulatory & Capital Matters (Details)	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Regulatory & Capital Matters
Tier 1 capital leverage ratio (as a percent)	9.09%	8.90%
Risk-based capital ratio (as a percent)	12.14%	12.29%
Old Second National Bank
Regulatory & Capital Matters
Tier 1 capital leverage ratio (as a percent)	10.23%	10.24%
Risk-based capital ratio (as a percent)	13.30%	13.45%
Tier One leverage ratio, basis point increase (as a percent)	0.01%
Risk-based capital ratio, basis point decrease (as a percent)	0.15%
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%