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Junior Subordinated Debentures - Issuance (Details) (USD $)
1 Months Ended 3 Months Ended 1 Months Ended
Jul. 31, 2003
Jun. 30, 2003
Mar. 31, 2015
Apr. 30, 2007
Dec. 31, 2014
Junior subordinated debentures          
Junior subordinated debentures     58,378,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust   $ 58,378,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Old Second Capital Trust I          
Junior subordinated debentures          
Proceeds from sale of cumulative trust preferred securities 4,100,000us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIMember
27,500,000us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIMember
     
Maturity Period     30 years    
Cash distribution rate of trust preferred securities (as a percent)     7.80%osbc_TrustPreferredSecuritiesCashDistributionRatePercentage
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIMember
   
Old Second Capital Trust II          
Junior subordinated debentures          
Proceeds from sale of cumulative trust preferred securities       25,000,000us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIIMember
 
Maturity Period     30 years    
Cash distribution fixed rate of trust preferred securities (as a percent)     6.77%osbc_TrustPreferredSecuritiesCashDistributionFixedRatePercentage
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIIMember
   
Basis points added to cash distribution floating rate base (as a percent)     1.50%osbc_TrustPreferredSecuritiesBasisSpreadOnVariableRate
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIIMember
   
Cash distribution, floating rate base     three-month LIBOR    
Junior subordinated debentures | Old Second Capital Trust I          
Junior subordinated debentures          
Junior subordinated debentures     32,600,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
   
Junior subordinated debentures | Old Second Capital Trust II          
Junior subordinated debentures          
Junior subordinated debentures     25,800,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
/ dei_LegalEntityAxis
= osbc_OldSecondCapitalTrustIIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember