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Junior Subordinated Debentures - Payments and other disclosures (Details) (USD $) | 0 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Apr. 21, 2014 | Dec. 31, 2014 item | Dec. 31, 2013 | Aug. 31, 2010
Junior subordinated debentures |  |  |  |
Junior subordinated debentures |  | 58,378,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust | $ 58,378,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust |
Payments to indentured trustees | 19,700,000 osbc_PaymentsForAccruedAndUnpaidInterestOnOutstandingTrustPreferredSecurities |  |  |
Junior subordinated debentures |  |  |  |
Junior subordinated debentures |  |  |  |
Junior subordinated debentures |  |  |  | 58,400,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_LongtermDebtTypeAxis = us-gaap_JuniorSubordinatedDebtMember
Period of interest payments that may be deferred (in quarters) |  | 20 osbc_InterestPaymentDefermentPeriod / us-gaap_LongtermDebtTypeAxis = us-gaap_JuniorSubordinatedDebtMember |  |
Junior subordinated debentures | Old Second Capital Trust I |  |  |  |
Junior subordinated debentures |  |  |  |
Junior subordinated debentures |  | 32,600,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_LongtermDebtTypeAxis = us-gaap_JuniorSubordinatedDebtMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = osbc_OldSecondCapitalTrustIMember |  |
Junior subordinated debentures | Old Second Capital Trust II |  |  |  |
Junior subordinated debentures |  |  |  |
Junior subordinated debentures |  | 25,800,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_LongtermDebtTypeAxis = us-gaap_JuniorSubordinatedDebtMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = osbc_OldSecondCapitalTrustIIMember |  |

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- Definition The period up to which the interest payments may be deferred. + References No definition available. + Details
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Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The cash outflow for accrued and unpaid interest on outstanding Trust Preferred Securities. + References No definition available. + Details
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Balance Type: | credit
Period Type: | duration

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- Details
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- Definition Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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Balance Type: | credit
Period Type: | instant

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- Details
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Balance Type: |
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