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Junior Subordinated Debentures - Issuance (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 12 Months Ended | 1 Months Ended
--- | --- | --- | ---
Jul. 31, 2003 | Jun. 30, 2003 | Dec. 31, 2014 | Apr. 30, 2007
Old Second Capital Trust I |  |  |  |
Junior subordinated debentures |  |  |  |
Proceeds from sale of cumulative trust preferred securities | $ 4.1 us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities / dei_LegalEntityAxis = osbc_OldSecondCapitalTrustIMember | $ 27.5 us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities / dei_LegalEntityAxis = osbc_OldSecondCapitalTrustIMember |  |
Amortization period |  |  | 30 years |
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 |  |  |  | $ 25.0 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 |
Cash distribution, floating rate base |  |  | three-month LIBOR |
Basis points added to cash distribution floating rate base (as a percent) |  |  | 1.50%osbc_TrustPreferredSecuritiesBasisSpreadOnVariableRate / dei_LegalEntityAxis = osbc_OldSecondCapitalTrustIIMember |

X
- 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

X
- 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 amortization period of the costs associated with the issuance of the trust preferred securities. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesIssuanceCostAmortizationPeriod
Namespace Prefix: | osbc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

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

X
- 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_
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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: |
Data Type: | na
Balance Type: |
Period Type: |