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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 |  |  | 58,378,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust |  | $ 58,378,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Old Second Capital Trust I |  |  |  |  |
Junior subordinated debentures |  |  |  |  |
Proceeds from sale of cumulative trust preferred securities | 4,100,000 us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities / dei_LegalEntityAxis = osbc_OldSecondCapitalTrustIMember | 27,500,000 us-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 |  |  |  | 25,000,000 us-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 |  |  | 32,600,000 us-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 |  |  | 25,800,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / dei_LegalEntityAxis = osbc_OldSecondCapitalTrustIIMember / us-gaap_LongtermDebtTypeAxis = us-gaap_JuniorSubordinatedDebtMember |  |

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- Definition The percentage points added to the reference rate to compute the variable rate on the trust preferred securities. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesBasisSpreadOnVariableRate
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the fixed annual rate of cash distributions on the securities. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesCashDistributionFixedRatePercentage
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the annual rate of cash distributions on the securities, which are payable quarterly. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesCashDistributionRatePercentage
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The reference rate for the variable rate of the trust preferred securities, such as LIBOR or the US treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesDescriptionOfVariableRateBasis
Namespace Prefix: | osbc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the maturity period of trust preferred securities. + References No definition available. + Details
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Namespace Prefix: | osbc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The cash inflow from issuance of preferred stocks by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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- Details
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