Regulatory & Capital Matters - Capital Levels and Industry Defined Regulatory Minimums (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Common equity tier 1 capital to risk weighted assets
Actual	$ 154,537	$ 151,410
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 90,411	$ 64,582
Common equity tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	8.76%	10.55%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	5.125%	4.50%
Minimum Required to Be Well Capitalized (as a percent)		6.50%
Total capital to risk weighted assets, Amount
Actual at period end	$ 216,769	$ 223,311
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 152,126	$ 114,813
Total capital to risk weighted assets, Ratio
Risk-based capital ratio (as a percent)	12.29%	15.56%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	8.625%	8.00%
Tier 1 capital to risk weighted assets, Amount
Actual	$ 191,988	$ 176,625
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 116,904	$ 86,159
Tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	10.88%	12.30%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	6.625%	6.00%
Tier 1 capital to average assets, Amount
Actual	$ 191,988	$ 176,625
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	$ 86,287	$ 81,300
Tier 1 capital to average assets, Ratio
Tier 1 capital leverage ratio (as a percent)	8.90%	8.69%
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	$ 221,153	$ 202,158
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	90,456	64,519
Minimum Required to Be Well Capitalized	$ 114,724	$ 93,193
Common equity tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	12.53%	14.10%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	5.125%
Minimum Required to Be Well Capitalized (as a percent)	6.50%
Total capital to risk weighted assets, Amount
Actual at period end	$ 237,306	$ 218,375
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	152,176	114,708
Minimum Required to Be Well Capitalized	$ 176,436	$ 143,385
Total capital to risk weighted assets, Ratio
Risk-based capital ratio (as a percent)	13.45%	15.23%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	8.625%	8.00%
Minimum Required to Be Well Capitalized (as a percent)	10.00%	10.00%
Tier 1 capital to risk weighted assets, Amount
Actual	$ 221,153	$ 202,158
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	116,930	86,025
Minimum Required to Be Well Capitalized	$ 141,199	$ 114,700
Tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	12.53%	14.10%
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable (as a percent)	6.625%	6.00%
Minimum Required to Be Well Capitalized (as a percent)	8.00%	8.00%
Tier 1 capital to average assets, Amount
Actual	$ 221,153	$ 202,158
Minimum Required for Capital Adequacy Purposes with Capital Conservation Buffer if applicable	86,388	81,351
Minimum Required to Be Well Capitalized	$ 107,985	$ 101,689
Tier 1 capital to average assets, Ratio
Tier 1 capital leverage ratio (as a percent)	10.24%	9.94%
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%