Regulatory & Capital Matters - Capital Levels and Industry Defined Regulatory Minimums (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Common equity tier 1 capital to risk weighted assets
Actual	$ 161,114	$ 154,537
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 108,352	$ 90,411
Common equity tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	8.55%	8.76%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	5.75%	5.125%
Total capital to risk weighted assets, Amount
Actual at period end	$ 228,754	$ 216,769
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 174,298	$ 152,126
Total capital to risk weighted assets, Ratio
Risk-based capital ratio (as a percent)	12.14%	12.29%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	9.25%	8.625%
Tier 1 capital to risk weighted assets, Amount
Actual	$ 208,397	$ 191,988
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 136,607	$ 116,904
Tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	11.06%	10.88%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	7.25%	6.625%
Tier 1 capital to average assets, Amount
Actual	$ 208,397	$ 191,988
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 91,704	$ 86,287
Tier 1 capital to average assets, Ratio
Tier 1 capital leverage ratio (as a percent)	9.09%	8.90%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	4.00%	4.00%
Old Second National Bank
Common equity tier 1 capital to risk weighted assets
Actual	$ 234,644	$ 221,153
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	108,283	90,456
Minimum Required to Be Well Capitalized	$ 122,407	$ 114,724
Common equity tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	12.46%	12.53%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	5.75%	5.125%
Minimum Required to Be Well Capitalized (as a percent)	6.50%	6.50%
Total capital to risk weighted assets, Amount
Actual at period end	$ 250,475	$ 237,306
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	174,203	152,176
Minimum Required to Be Well Capitalized	$ 188,327	$ 176,436
Total capital to risk weighted assets, Ratio
Risk-based capital ratio (as a percent)	13.30%	13.45%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	9.25%	8.625%
Minimum Required to Be Well Capitalized (as a percent)	10.00%	10.00%
Tier 1 capital to risk weighted assets, Amount
Actual	$ 234,644	$ 221,153
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	136,530	116,930
Minimum Required to Be Well Capitalized	$ 150,654	$ 141,199
Tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	12.46%	12.53%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	7.25%	6.625%
Minimum Required to Be Well Capitalized (as a percent)	8.00%	8.00%
Tier 1 capital to average assets, Amount
Actual	$ 234,644	$ 221,153
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	91,747	86,388
Minimum Required to Be Well Capitalized	$ 114,684	$ 107,985
Tier 1 capital to average assets, Ratio
Tier 1 capital leverage ratio (as a percent)	10.23%	10.24%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	4.00%	4.00%
Minimum Required to Be Well Capitalized (as a percent)	5.00%	5.00%