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Employee Benefit Plans (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Old Second Bancorp, Inc. Voluntary Deferred Compensation Plan for Executives |  |  |
Employee Benefit Plans |  |  |
Company obligations under Voluntary Deferred Compensation Plan for Executives | $ 1,900,000 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DeferredCompensationExcludingShareBasedPaymentsAndRetirementBenefitsMember | $ 1,800,000 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DeferredCompensationExcludingShareBasedPaymentsAndRetirementBenefitsMember | $ 1,500,000 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DeferredCompensationExcludingShareBasedPaymentsAndRetirementBenefitsMember
Old Second Bancorp Inc Employees 401(k) Savings Plan and Trust |  |  |
Employee Benefit Plans |  |  |
Company's discretionary matching contributions to the Plan (as a percent) | 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember
Percentage of compensation contributed to the Plan matched by employer | 2.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | 2.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | 2.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember
Participants vesting as percentage of discretionary matching contributions | 100.00%us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | 100.00%us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | 100.00%us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember
Expense relating to the 401(k) plan | $ 477,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | $ 468,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember | $ 441,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = osbc_OldSecondBancorpIncEmployees401KSavingsPlanAndTrustMember

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- Definition Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements. Represents currently earned compensation under compensation arrangements that is not actually paid until a later date. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of employer's matching contributions to a defined contribution plan that vests in a given year. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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Data Type: | na
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- Details
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Data Type: | na
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Period Type: |