Regulatory & Capital Matters - Capital Levels and Industry Defined Regulatory Minimums (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Common equity tier 1 capital to risk weighted assets
Actual	$ 136,976
Minimum Required for Capital Adequacy Purposes	65,158
Common equity tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	9.46%
Minimum Required for Capital Adequacy Purposes (as a percent)	4.50%
Total capital to risk weighted assets, Amount
Actual at period end	250,416us-gaap_Capital	240,566us-gaap_Capital
Minimum Required for Capital Adequacy Purposes	115,799	108,853
Total capital to risk weighted assets, Ratio
Risk-based capital ratio (as a percent)	17.30%us-gaap_CapitalToRiskWeightedAssets	17.68%us-gaap_CapitalToRiskWeightedAssets
Minimum Required for Capital Adequacy Purposes (as a percent)	8.00%	8.00%
Tier 1 capital to risk weighted assets, Amount
Actual	197,987us-gaap_TierOneRiskBasedCapital	196,499us-gaap_TierOneRiskBasedCapital
Minimum Required for Capital Adequacy Purposes	86,836	54,432
Tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	13.68%	14.44%
Minimum Required for Capital Adequacy Purposes (as a percent)	6.00%	4.00%
Tier 1 capital to average assets, Amount
Actual	197,987us-gaap_TierOneLeverageCapital	196,499us-gaap_TierOneLeverageCapital
Minimum Required for Capital Adequacy Purposes	80,646	79,154
Tier 1 capital to average assets, Ratio
Tier 1 capital leverage ratio (as a percent)	9.82%	9.93%
Minimum Required for Capital Adequacy Purposes (as a percent)	4.00%	4.00%
Old Second National Bank
Common equity tier 1 capital to risk weighted assets
Actual	245,076
Minimum Required for Capital Adequacy Purposes	65,218
Minimum Required to Be Well Capitalized	94,204
Common equity tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	16.91%
Minimum Required for Capital Adequacy Purposes (as a percent)	4.50%
Minimum Required to Be Well Capitalized (as a percent)	6.50%
Total capital to risk weighted assets, Amount
Actual at period end	263,398	254,897
Minimum Required for Capital Adequacy Purposes	115,907	108,872
Minimum Required to Be Well Capitalized	144,883	136,090
Total capital to risk weighted assets, Ratio
Risk-based capital ratio (as a percent)	18.18%	18.73%
Minimum Required for Capital Adequacy Purposes (as a percent)	8.00%	8.00%
Minimum Required to Be Well Capitalized (as a percent)	10.00%	10.00%
Tier 1 capital to risk weighted assets, Amount
Actual	245,076	237,828
Minimum Required for Capital Adequacy Purposes	86,958	54,454
Minimum Required to Be Well Capitalized	115,944	81,681
Tier 1 capital to risk weighted assets, Ratio
Actual (as a percent)	16.91%	17.47%
Minimum Required for Capital Adequacy Purposes (as a percent)	6.00%	4.00%
Minimum Required to Be Well Capitalized (as a percent)	8.00%	6.00%
Tier 1 capital to average assets, Amount
Actual	245,076	237,828
Minimum Required for Capital Adequacy Purposes	80,683	79,144
Minimum Required to Be Well Capitalized	$ 100,854	$ 98,930
Tier 1 capital to average assets, Ratio
Tier 1 capital leverage ratio (as a percent)	12.15%	12.02%
Minimum Required for Capital Adequacy Purposes (as a percent)	4.00%	4.00%
Minimum Required to Be Well Capitalized (as a percent)	5.00%	5.00%

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Tier 1 Risk Based Capital, as defined in the regulations, attributable to common shares.

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Tier 1 capital attributable to common equity divided by risk weighted assets as defined by regulations.

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The minimum amount of Tier 1 Risk Based Common Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Tier One Common Capital Ratio (Tier one common capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of total capital as defined in the regulations.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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